1. Social nature of human beings
God did not create man as a “solitary being," but wanted him to be a “social being" (cf. Gen 1:27; 2:18-20, 23). Life in society is not something “added-on" for the human person; it stems from an important dimension intrinsic to human nature. Human beings can grow and attain their calling in life only in union with others. 
The social nature of the human person is even more evident in the light of faith, since there is a certain likeness between the intimate life of the Holy Trinity and the communion that should be established among mankind; all men and women have been redeemed by Christ and are called to share in the same end.  Revelation teaches us that human relationships should be marked by a deep gratuitousness, since in our neighbor we see, more than an equal, the living image of God, and we have to be ready to give ourselves fully, even to the ultimate extreme, for each one. 
Therefore each person “is called to exist 'for' others, to become a gift."  Existing “for" others entails much more than just “co-existing" with them: it means serving them and loving them. Human freedom “can wither in an ivory-tower isolation brought on by an over indulgence in the good things of life." 
A personal and collective effort is needed to develop social relationships correctly: “ The social nature of human beings does not automatically lead to communion among persons, to the gift of self . Because of pride and selfishness, man discovers in himself the seeds of asocial behavior, impulses leading him to close himself within his own individuality and to dominate his neighbor." 
The person's social nature is not limited to political and commercial aspects; the relationships based on the most deeply human aspects of the person, especially the spiritual ones, are much more important for the correct building up of society.  Therefore the real possibility of constructing a society worthy of the human person depends on the interior growth of men and women. It is free human acts that build up a society, and not the impersonal forces of history. Hence the special importance that should be accorded to spiritual values and human relationships based on sincere self-giving.
The social nature of the human person entails another important characteristic: the radical equality of all human beings. All men and women possess the same human nature and origin; all have been redeemed by Christ and called to share in the same divine happiness.
“All therefore enjoy an equal dignity" ( Catechism , 1934). Along with this radical equality there are also differences, which have a positive value if they are not the result of injustice: “These differences belong to God's plan, who wills that each receive what he needs from others, and that those endowed with particular 'talents' share the benefits with those who need them" (Catechism, 1937).
The social nature of the human being finds expression in the setting up of a variety of associations aimed at attaining distinct goals: “A society is a group of persons bound together organically by a principle of unity that goes beyond each one of them" (Catechism, 1880).
Many different human goals exist, as well as bonds that unite people: love, ethnicity, language, country, culture, etc. Therefore human relationships entail a wide mosaic of institutions and associations: the family, city, state, international community, etc.
Certain societies, such and the family and state, correspond more directly to human nature and therefore are necessary. Others correspond to people's free initiative, and are intrinsic to what could be termed human “socialization." This “socialization" expresses the natural tendency for human beings to associate with one another for the sake of attaining objectives that exceed individual capacities, and help develop the human qualities of each person (cf. Catechism, 1882).
The close tie between the person and life in society explains the enormous influence of society on the development of each person, and the harm that is done to the human person by living in a defectively organized society. The way people behave depends, to some extent, on the way society is organized, which exerts a cultural influence on people.
While never reducing the human being to an anonymous element in society,  it is good to remember that the integral development of the human person and social progress mutually influence one another.  No opposition exists between the personal dimension and the social dimension of the human being. Rather these two dimensions are intimately united and are strengthened in union with one another.
In this context, due to people's sins, unjust structures or “structure of sin" can be set up.  These structures are opposed to the right ordering of society, and make the practice of virtue more difficult and facilitate personal sins against justice, charity, chastity, etc. These structures may involve widespread immoral customs (for example, political and economic corruption), or unjust laws (such as those allowing abortion), etc.  A serious effort should be made to undo these “structures of sin" and replace them with just structures. 
To undo unjust structures and christianize professional relationships and the entire society, the diligent effort to live the moral norms intrinsic to one's professional work is very important. This is also a necessary condition for sanctifying that work.
“Every human community needs an authority to govern it. The foundation of such authority lies in human nature. It is necessary for the unity of the state. Its role is to ensure as far as possible the common good of the society" ( Catechism , 1898).
Since our social nature is a property of human nature, all legitimate authority stems from God, the Author of nature (cf. Rom 13:1; Catechism , 1899). However, “the choice of the political regime and the appointment of rulers are left to the free decision of the citizens." 
The moral legitimacy of authority does not stem from itself, but rather from being God's servant (cf. Rom 13:4), and is ordered to the common good.  Those who have been given authority should exercise it as a service, practice distributive justice, avoid favoritism and self-seeking, and never act despotically (cf. Catechism , 1902, 2235 and 2236).
“While public authority can sometimes choose not to put a stop to something which—were it prohibited—would cause more serious harm (cf. St. Thomas Aquinas, Summa Theologiae, I-II, q. 96, a. 2), it can never presume to legitimize as a right of individuals—even if they are the majority of the members of society—an offence against other persons caused by the disregard of so fundamental a right as the right to life." 
With regard to political systems, “the Church values the democratic system inasmuch as it ensures the participation of citizens in making political choices, guarantees to the governed the possibility both of electing and holding accountable those who govern them."  That a state be a democracy is part of the common good. But “the value of democracy stands or falls with the values which it embodies and promotes. Of course, values such as the dignity of every human person, respect for inviolable and inalienable human rights . . . are certainly fundamental and not to be ignored."  “A democracy without values easily turns into open or thinly disguised totalitarianism." 
4. The common good
The common good can be defined as “the sum total of social conditions which allow people, either as groups or as individuals, to reach their fulfillment more fully and more easily."  Therefore the common good is not only a matter of material well-being but also spiritual (the two are inter-connected), and entails “three essential elements" ( Catechism , 1906):
—respecting the human person and his or her freedom  ;
—furthering social progress and integral human development  ;
—fostering “peace, that is, the stability and security of a just order" (Catechism , 1909). 
Given the human being's social nature, the good of each person is necessarily related to the common good, which in turn should be directed to each one's progress (cf. Catechism , 1905 and 1912). 
The scope of the common good is not limited to a city or country. A “universal common good" also exists, which “calls for an organization of the community of nations" (Catechism, 1911).
5. Society and the human person's transcendent dimension
The social nature of the person encompasses all human dimensions, including the transcendent one. The deepest truth about the person, and the ground of all human dignity, is being created in the image and likeness of God and called to communion with Him.  “Christian anthropology therefore is really a chapter of theology . . . The theological dimension is needed both for interpreting and solving present-day problems in human society." 
Hence the emptiness of social projects that overlook this transcendent dimension. Atheism, in its various manifestations, has many harmful effects on society.  This reality is especially evident today. In the measure that the religious roots of a community are lost sight of, inter-personal relationships become more tense and violent, since the moral force needed to act rightly is weakened or even lost. 
A stable social order requires an unwavering foundation that is not at the mercy of changing opinions or the hunger for power; only belief in God can provide an unshakable foundation for society.  Hence the importance of not separating, or even worse placing in opposition, the religious and social dimensions of the human person.  Both aspects about the truth of the human person need to be harmoniously fostered: the sincere search for God (cf. Catechism , 358 and 1721; Compendium , 109) and the concern for one's neighbor and the progress of society, a concern that is strengthened by the theocentric dimension. 
Hence the indispensable need for spiritual growth in order to further society's development: social renewal is nourished by contemplation. The encounter with God in prayer introduces a mysterious force into history that changes hearts and moves people to convert, thus providing the energy needed to transform social structures.
To strive to bring about social change without a serious effort to change one's own life only leads to disillusionment in the end, and often even lowers the level of people's lives.
A “new social order" that is realistic, and therefore always open to improvement, requires the growth in the needed technical and scientific skills,  but also at the same time the strengthening of people's moral formation and spiritual life. This is the true path for renewing society's institutions and structures. 
Nor should one forget that the effort to construct a just social order ennobles the person who endeavors to bring this about.
6. Participation by Catholics in public life
The effort to foster the common good, each in one's own place and role in society, is a duty “inherent in the dignity of the human person" (Catechism, 1913). No one can accept “the luxury of a merely individualistic morality."  Therefore “as far as possible citizens should take an active part in public life" (Catechism, 1915). 
The right and duty to take part in the life of society stems from the principle of subsidiarity: “a community of a higher order should not interfere in the internal life of a community of a lower order, depriving the latter of its functions, but rather should support it in case of need and help to coordinate its activity with the activities of the rest of society, always with a view to the common good." 
This participation in public life requires, first of all, the responsible fulfillment of one's family and professional duties (cf. Catechism, 1914) and the obligations of legal justice (for example, paying taxes).  It also means practicing the human virtues, especially solidarity with others.
Given the interdependence of persons and human groups, participation in public life should be carried out with a spirit of solidarity, striving for the good of others.  “Socio‑economic problems can be resolved only with the help of all the forms of solidarity: solidarity of the poor among themselves, between rich and poor, of workers among themselves, between employers and employees in a business, solidarity among nations and peoples. International solidarity is a requirement of the moral order; world peace depends in part upon this" (Catechism, 1941).  Although solidarity should encompass all men and women, “the duty of making oneself a neighbor to others and actively serving them becomes even more urgent when it involves the disadvantaged, in whatever area this may be" (Catechism, 1932; cf. 2443-2449).
As citizens, the faithful have the same rights and duties are their fellow citizens; but as Catholics, they bear a special responsibility (cf. Tit 3:1-2; 1 Pet 2:13-15).  Therefore the lay faithful can never abdicate their responsibility to take part in political life.  This participation is particularly needed in order to permeate “social, political, and economic realities with the demands of Christian doctrine and life" (Catechism, 899).
Given the reality that civil laws often fail to accord with the Church's teaching, Catholics should do all they can, with the help of other citizens of good will, to rectify these law, always acting within legitimate channels and with charity.  In any case, their conduct should always be guided by Catholic teaching, even when society may present obstacles, since we must obey God rather than men (Acts 5:29).
In summary, Catholics should diligently exercise their civil rights and fulfill their duties. This falls especially to the lay faithful, who are called to sanctify the world from within, with personal initiative and responsibility, without waiting for the hierarchy to resolve problems with civil authorities or to indicate the solutions they should adopt. 
Catechism of the Catholic Church , 1877-1917; 1939-1942; 2234-2249.
Compendium of the Social Doctrine of the Church , 34-43; 149-151; 164-170; 541-574.
St. Josemaria, Homily “Christ the King," in Christ is Passing By , 179-187.
Congregation for the Doctrine of the Faith, Doctrinal Note on Some Questions Regarding the Participation of Catholics in Political Life , 24 November 2002.
 Cf. Vatican II, Gaudium et spes , 24-25; Congregation for the Doctrine of the Faith,
Instr. Libertatis conscientia , 32; Compendium of the Social Doctrine of the Church , 110.
 “Being in communion with Jesus Christ draws us into his 'being for all'; it makes it our own way of being. He commits us to live for others, but only through communion with him does it become possible truly to be there for others" (Benedict XVI, Enc. Spe salvi , 30 November 2007, 28).
 Cf. John Paul II, Enc. Sollicitudo rei socialis , 30 December1987, 40.
 John Paul II, Apost. Letter Mulieris dignitatem , 15 August 1988, 7.
 Vatican II, Gaudium et spes , 31.
 Compendium of the Social Doctrine of the Church , 150.
 Cf. Benedict XVI, Spe salvi , 24.
 “The origins of a society existing in history are found in the interconnectedness of the freedoms of all the persons who interact within it, contributing by means of their choices either to build it up or to impoverish it" ( Compendium of the Social Doctrine of the Church , 163).
 The human person is “the beginning, the subject and the object of every social organization" ( Gaudium et spes , 25). Cf. Pius XII, Radio Message for Christmas, 24 December1942: AAS 35 (1943) 12; John XXIII, Enc. Mater et magistra : AAS 53 (1961) 453; Compendium of the Social Doctrine of the Church , 106.
 Cf. John Paul II, Enc. Sollicitudo rei socialis , 38; Catechism , 1888; Compendium of the Social Doctrine of the Church , 62, 82 and 134.
 Cf. John Paul II, Enc. Sollicitudo rei socialis , 36.
 “Whenever the Church speaks of situations of sin or when she condemns as social sins certain situations or the collective behavior of certain social groups . . . she knows and she proclaims that such cases of social sin are the result of the accumulation and concentration of many personal sins. It is a case of the very personal sins of those who cause or support evil or who exploit it; of those who are in a position to avoid, eliminate or at least limit certain social evils but who fail to do so out of laziness, fear or the conspiracy of silence, through secret complicity or indifference; of those who take refuge in the supposed impossibility of changing the world and also of those who sidestep the effort and sacrifice required" (John Paul II, Apost. Exhort. Reconciliatio et paenitentia , 2 December 1984, 16.
 Cf. Vatican II, Const. Lumen gentium , 36; John Paul II, Enc. Centesimus annus , 38; Compendium of the Social Doctrine of the Church , 570. This effort will usually involve a process and not an immediate change, which means that the faithful will often have to “co-exist" with these structures and endure their consequences, without being corrupted by them or ceding in the effort to change them. Our Lord's words are very relevant here: I do not pray that thou shouldst take them out of the world, but that thou shoudst keep them from the evil one (Jn 17:15).
 Gaudium et spes , 74. Cf. Catechism , 1901.
 “Authority is exercised legitimately only when it seeks the common good of the group concerned and if it employs morally licit means to attain it. If rulers were to enact unjust laws or take measures contrary to the moral order, such arrangements would not be binding in conscience" (Catechism, 1903).
 John Paul II, Enc. Evangelium vitae , 25 March 1995, 71.
 John Paul II, Enc. Centesimus annus , 46.
 John Paul II, Enc. Evangelium vitae , 70. The Pope refers in particular to the right of each innocent human being to life, which laws permitting abortion violate.
 John Paul II, Centesimus annus , 46.
 Gaudium et spes , 26. Cf. Catechism , 1906.
 “In the name of the common good, public authorities are bound to respect the fundamental and inalienable rights of the human person. Society should permit each of its members to fulfill his vocation" (Catechism, 1907).
 The public authority, while respecting the principle of subsidiarity and fostering private initiative, should strive to ensure that each person possesses what is needed to lead a humanly dignified life: food, clothing, health, work, education and culture, etc. Cf. Catechism , 1908 and 2211.
 Peace is not only the absence of war. Peace cannot be attained without safeguarding the dignity of the human person (cf. Catechism , 2304). Peace is the “tranquility of order" (St. Augustine, De civitate Dei , 19, 13). It is a work of justice (cf. Is 32:17). “The common good requires peace, that is, the stability and security of a just order. It presupposes that authority should ensure by morally acceptable means the security of society and its members. It is the basis of the right to legitimate personal and collective defense" (Catechism, 1909).
 The common good is always oriented towards the progress of persons: “The order of things must be subordinate to the order of persons, and not the other way around" (Gaudium et spes , 26).
 Cf. Gaudium et spes, 19.
 John Paul, II, Enc. Centisimus annus, 55. Cf. Gaudium et spes, 11 and 41.
 Cf. John Paul II, Enc. Evangelium vitae, 21-24. “If we then inquire as to the source of this mistaken concept of the nature of the person and the 'subjectivity' of society, we must reply that its first cause is atheism. It is by responding to the call of God contained in the being of things that man becomes aware of his transcendent dignity . . . The denial of God deprives the person of his foundation, and consequently leads to a reorganization of the social order without reference to the person's dignity and responsibility" (John Paul II, Centesimus annus, 13).
 “Man can set about organizing terrestrial realities without God. But closed off from God, they will end up being directed against man" (Paul VI, Enc. Populorum progressio , 26 March 1967, 42). Cf. John XXIII, Enc. Mater et magistra : AAS 53 (1961) 452-453; Vatican II, Const. Gaudium et spes , 21; Benedict XVI, Enc. Deus caritas est , 25 December 2005, 42.
 Cf. Leo XIII, Enc. Diuturnum illud : Acta Leonis XIII, 2 (1882) 277 and 278; Pius XI, Enc. Caritate Christi : AAS 24 (1932) 183-184.
 “Some people tend to see Christianity as a collection of devout practices, failing to realize the relation between them and the circumstances of ordinary life, including the urgency to meet the needs of other people and remedy injustice . . . Others tend to imagine that in order to remain human we need to play down some central aspects of Christian dogma. They act as if the life of prayer, continual relationship with God, implied fleeing from responsibilities and forsaking the world. But they forget that it was none other than Jesus who showed us the extreme to which we should go in love and service. Only if we try to understand the mystery of God's love—a love which went as far as death—will we be able to give ourselves totally to others and not let ourselves be overcome by difficulties or indifference" (St. Josemaria, Christ is Passing By , 98).
 There is a deep inter-connection “between love of God and love of neighbor . . . If I have no contact whatsoever with God in my life, then I cannot see in the other anything more than the other, and I am incapable of seeing in him the image of God. But if in my life I fail completely to heed others, seeking only to be 'devout' and to perform my 'religious duties,' then my relationship with God will also grow arid" (Benedict XVI, Enc. Deus caritas est , 18). Cf. John Paul II, Enc. Evangelium vitae , 35-36; Compendium of the Social Doctrine of the Church , 40.
 “All professional work demands previous training and a constant effort to improve one's formation and to adapt it to the new circumstances that may arise. And this is very specially true for those who aspire to occupy leading positions in society, because they are called to a very important service on which the entire community's well‑being depends" (St. Josemaria, Conversations, 90).
 “We contribute to a better world only by personally doing good now, with full commitment and wherever we have the opportunity" (Benedict XVI, Enc. Deus caritas est , 31b).
 Gaudium et spes, 30.
 “A person or a society that does not react to suffering and injustice and makes no effort to alleviate them is still distant from the love of Christ's heart. While Christians enjoy the fullest freedom in finding and applying various solutions to these problems, they should be united in having one and the same desire to serve mankind. Otherwise their Christianity will not be the word and life of Jesus; it will be a fraud, a deception of God and man" (St. Josemaria, Christ is Passing By , 167).
 John Paul II, Enc. Centesimus annus , 48. Cf. Catechism , 1883; Compendium of the Social Doctrine of the Church, 186 and 187.
“The principle of subsidiarity is opposed to all forms of collectivism. It sets limits for state intervention. It aims at harmonizing the relationships between individuals and societies. It tends toward the establishment of true international order" (Catechism , 1885).
God “entrusts to every creature the functions it is capable of performing, according to the capacities of its own nature. This mode of governance ought to be followed in social life. The way God acts in governing the world, which bears witness to such great regard for human freedom, should inspire the wisdom of those who govern human communities. They should behave as ministers of divine providence" (Catechism , 1884).
 Legal justice “concerns what the citizen owes in fairness to the community" ( Catechism , 2411). “Submission to authority and co‑responsibility for the common good make it morally obligatory to pay taxes" (Catechism, 2240). “Fraud and other subterfuges, by which some people evade the constraints of the law and the prescriptions of societal obligation, must be firmly condemned because they are incompatible with the requirements of justice" (Catechism, 1916).
 “It is above all a question of interdependence, sensed as a system determining relationships in the contemporary world, in its economic, cultural, political and religious elements, and accepted as a moral category. When interdependence becomes recognized in this way, the correlative response as a moral and social attitude, as a 'virtue,' is solidarity" (John Paul II, Enc. Sollicitudo rei socialis, 38).
 Cf. Compendium of the Social Doctrine of the Church, 193-195.
 Cf. Gaudium et spes, 75.
 Cf. John Paul II, Christifideles laici, 42.
 For example, “when it is not possible to overturn or completely abrogate a pro-abortion law, an elected official, whose absolute personal opposition to procured abortion was well known, could licitly support proposals aimed at limiting the harm done by such a law and at lessening its negative consequences at the level of general opinion and public morality" (John Paul II, Enc. Evangelium vitae, 73).
 “The laity have the duty of using their own initiative and taking action in this area—without waiting passively for directives and precepts from others. They must try to infuse a Christian spirit into people's mental outlook and daily behavior, into the laws and structures of the civil government “(Paul VI, Enc. Populorum progressio, 81). Cf. Lumen gentium, 31; Gaudium et spes, 43; John Paul II, Christifideles laici, 15; Catechism , 2442.