1. The Incarnation of the Word
But when the time had fully come, God sent forth his Son, born of woman (Gal 4:4). Thus the promise of a Saviour that God had made to Adam and Eve as they were expelled from Paradise was fulfilled: I will put enmity between you and the woman, and between your seed and her seed; he shall bruise your head and you shall bruise his heel (Gen 3:15). This verse from Genesis is sometimes called the “proto-gospel" or first gospel, because it is the first announcement of the good news of salvation. The traditional interpretation is that the “woman" of whom it speaks is both Eve, in a direct sense, and Mary in the full sense; and that the “seed" of the woman refers both to mankind and to Christ.
From then until the moment when the word became flesh and dwelt among us (Jn 1:14), God was preparing mankind to welcome his only-begotten Son. God chose the people of Israel for himself, established his Covenant with them, and formed them progressively, intervening in their history, telling them his plans through the patriarchs and prophets, and sanctifying them for himself. All this was a preparation and figure of the new and perfect Covenant that was to be forged in Christ, and of the full and definitive revelation that was to be brought about by the Incarnate Word himself. Although God prepared the coming of the Saviour above all by choosing the people of Israel, this does not mean that he abandoned other people, “the Gentiles," for he never ceased giving them testimony of himself (cf. Acts 14:16-17). Divine providence ensured that the Gentiles had some degree of awareness of the need for salvation, and the desire to be redeemed stretched to the very ends of the earth.
The origin of the Incarnation is God's love for mankind. In this the love of God was made manifest among us, that God sent his only Son into the world, so that we might live through him ( 1 Jn 4:9). The Incarnation is the supreme sign of God's love for us, since God gives himself to us through the Second Person of the Blessed Trinity coming to share in our human nature in the unity of the Son's divine Person.
After the fall of Adam and Eve in paradise, the Incarnation has a saving and redemptive purpose, as we profess in the Creed. “For us men and for our salvation he came down from heaven: by the power of the Holy Spirit he was born of the Virgin Mary, and became man."  Christ said of himself that the Son of man came to seek and to save what was lost ( Lk 19:10; cf. Mt 18:11), and that God sent his Son into the world not to condemn the world, but that the world might be saved by him ( Jn 3:17).
The Incarnation not only shows God's infinite love for mankind, his infinite mercy, justice and power, but also the divine wisdom shown in the way God decided to save man, which is the way that was most appropriate to human nature: through the Incarnation of the Word.
Jesus Christ, the Incarnate Word, “is not a myth, or an abstract idea; he is a man who lived in a specific context and who died after a life spent on earth in the course of history. Historical research about him is, therefore, required by Christian faith." 
That Christ existed belongs to the doctrine of faith, as also that he really died for us and rose on the third day (cf. 1 Cor 15:3-11). Christ's existence is a fact proved by history, particularly by the analysis of the New Testament, whose historical value is beyond doubt. We have other ancient non-Christian testimonies, both pagan and Jewish, about Christ's life. Precisely because of this we cannot accept the position of those who set up a “historical Jesus" in opposition to the “Christ of faith," and who defend the supposition that almost everything the New Testament says about Christ is an interpretation of faith made by Jesus' disciples, but not his true historical figure, which remains hidden from us. These points of view, which often include a strong prejudice against anything supernatural, fail to account for the fact, confirmed by contemporary historical research, that the representation of Christ offered by early Christian witnesses is underpinned by events that really took place.
2. Jesus Christ, true God and true man
The Incarnation “is the mystery of the wonderful union of the divine and human natures in the one person of the Word" (CCC, 483). The Incarnation of the Son of God “does not mean that Jesus Christ is part God and part man, nor does it imply that he is the result of a confused mixture of the divine and the human. He became truly man while remaining truly God. Jesus Christ is true God and true man" (CCC, 464). The divinity of Jesus Christ, the eternal Word of God, was dealt with in summary no. 5 on the Blessed Trinity. Here we will focus primarily on his humanity.
The Church defended and clarified this truth of faith during the first centuries against the heresies which denied or misrepresented it. As far back as the first century some Christians of Jewish origin, the Ebionites, held that Christ was simply a man, although a very holy man. “Adoptionism" arose in the second century, maintaining that Jesus was the adopted son of God: that Jesus was only a man in whom God's strength dwelt. According to this heresy, God was one single person. It was condemned by Pope St Victor in 190 A.D., by the Council of Antioch in 268, by the First Council of Constantinople and by the Roman Synod of 382.  The Arian heresy, by denying the divinity of the Word, also denied that Jesus Christ was God. Arius was condemned by the Council of Nicaea in the year 325. Today the Church has again reminded us that Jesus Christ is the Son of God subsistent from all eternity and that in the Incarnation he assumed human nature in his one divine Person.
The Church also confronted other errors that denied the reality of Christ's human nature. These included heresies that rejected the reality of Christ's body or of his soul. Amongst the former were various forms of docetism, which has a Gnostic and Manichean background. Some of its followers held that Christ had a celestial body, or that his body was merely apparent, or that he suddenly appeared in Judaea without having been born or grown up. St John already had to combat this error: for many deceivers have gone out into the world, men who will not acknowledge the coming of Jesus Christ in the flesh ( 2 Jn 7; cf. 1 Jn 4:1-1).
Arius and Apollinarius of Laodicea denied that Christ had a true human soul. The latter was particularly important in spreading this error and his influence was felt for several centuries in the later Christological controversies. In an attempt to defend Christ's unity and impeccability, Apollinarius maintained that the Word fulfilled the functions of the human spiritual soul. This doctrine, however, meant a denial of Christ's true humanity, composed, as in all men, of body and spiritual soul (cf. CCC 471). He was condemned in the First Council of Constantinople and the Roman Synod of 382. 
3. The Hypostatic Union
At the beginning of the fifth century, after the preceding controversies, there was a clear need to firmly defend the integrity of the two natures, human and divine, in the one Person of the Word. Thus the personal unity of Christ became the centre of attention of patristic Christology and soteriology. New discussions contributed to this new depth of understanding.
The first great controversy originated with some statements by Nestorius, patriarch of Constantinople, who implied that in Christ there are two subjects: the divine subject and the human subject, united by a moral bond, but not physically. This Christological error was the origin of his rejection of the title of Mother of God, Theotókos, applied to our Lady. According to his view, Mary would be the Mother of Christ, but not the Mother of God. Against this heresy, St Cyril of Alexandria and the Council of Ephesus in 431 stressed that “Christ's humanity has no other subject than the divine person of the Son of God, who assumed it and made it his own from his conception. For this reason the Council of Ephesus proclaimed in 431 that Mary truly became the Mother of God by the human conception of the Son of God in her womb" (CCC, 466; DS 250 and 252).
Some years later the Monophysite heresy arose. This heresy has antecedents in Apollinarianism and a misunderstanding of St. Cyril's teaching and language by Eutyches, an elderly archimandrite in a monastery in Constantinople. Eutyches affirmed, amongst other things, that Christ was a Person who subsisted in a single nature, since his human nature would have been absorbed into his divine nature. This error was condemned by Pope Leo the Great, in his Tomus ad Flavianum, a real jewel of Latin theology, and by the Ecumenical Council of Chalcedon in 451, a necessary reference-point for Christology. This Council teaches that “we confess one and the same Son, our Lord Jesus Christ: perfect in divinity and perfect in humanity." It adds that the union of the two natures is “without confusion, change, division or separation." 
The doctrine of Chalcedon was confirmed and clarified in the year 553 by the Second Council of Constantinople, which offered an authentic interpretation of the previous Council. After repeatedly emphasising the unity of Christ,  it affirmed that the union of the two natures in Christ takes place by hypostasis.  In this way it overcame the ambiguity of St Cyril's formula, which spoke of unity according to “ physis. " The Second Council of Constantinople also indicated the true sense of St Cyril's well-known formula, “one incarnate nature of the Word of God"  (a phrase that St Cyril thought came from St Athanasius, but which was in fact an Apollinarian falsification).
In these conciliar definitions, which aimed to clarify specific errors and not to expound the mystery of Christ in its totality, the Council Fathers used the language of their time. Just as Nicaea used the term “consubstantial," Chalcedon used terms such as nature, person, hypostasis, etc., following the usual meaning that they had in ordinary language and in the theology of the time. This does not mean, as some have affirmed, that the Gospel message became hellenised. In reality, those who showed themselves to be rigidly hellenist were precisely those who proposed heretical doctrines, such as Arius or Nestorius, who could not see the limitations of the philosophical language of their time when trying to describe the mystery of God and of Christ.
4. Christ's Sacred Humanity
“In the Incarnation 'human nature was assumed, not absorbed' (GS 22, 2)" (CCC, 470). Therefore the Church defends “the full reality of Christ's human soul, with its operations of intellect and will, and of his human body. In parallel fashion, she had to recall on each occasion that Christ's human nature belongs, as his own, to the divine person of the Son of God, who assumed it. Everything that Christ is and does in this nature derives from 'one of the Trinity.' The Son of God therefore communicates to his humanity his own personal mode of existence in the Trinity. In his soul as in his body, Christ thus expresses humanly the divine ways of the Trinity (cf. Jn 14:9-10)" (CCC, 470).
Christ's human soul possesses true human knowledge. Catholic doctrine has traditionally taught that, as man, Christ possessed acquired knowledge, infused knowledge, and the knowledge proper to the blessed in heaven. Christ's acquired knowledge could not in itself be unlimited. “This is why the Son of God could, when he became man, 'increase in wisdom and in stature, and in favour with God and man' (Lk 2:52), and would even have to inquire for himself about what one in the human condition can learn only from experience (cf. Mk 6:38; 8:27; Jn 11:34)" (CCC, 472). Christ, in whom the fullness of the Holy Spirit dwells with his gifts (cf. Is 11:1-3), also possesses infused knowledge, that is, knowledge that is not acquired directly by the work of the reason, but is infused directly by God in the human intellect. Thus, “the Son in his human knowledge also showed the divine penetration he had into the secret thoughts of human hearts (cf. Mk 2:8; Jn 2:25; 6:61)" (CCC, 473). Christ also possesses the knowledge proper to the blessed: “By its union to the divine wisdom in the person of the Word incarnate, Christ enjoyed in his human knowledge the fullness of understanding of the eternal plans he had come to reveal (cf. Mk 8:31; 9:31; 10:33-34; 14:18-20, 26-30)" (CCC 474).
For all these reasons it must be stated that Christ as man is infallible: to admit error in him would be to admit it in the Word, the one Person existing in Christ. With regard to ignorance as such, we have to bear in mind that “what he admitted to not knowing in this area, he elsewhere declared himself not sent to reveal (cf. Acts 1:7)" (CCC, 474). We can understand that, on the human plane, Christ was aware of being the Word and of his saving mission.  On the other hand, Catholic theology, in view of the fact that while on earth Christ already possessed the immediate vision of God, has always denied that the virtue of faith existed in Christ. 
Against the monoenergetic and monothelitic heresies which, following logically from the preceding monophysicism, affirmed that in Christ there is a single operation or a single will, the Church confessed in the third Ecumenical Council of Constantinople, in the year 681, that “Christ possesses two wills and two natural operations, divine and human. They are not opposed to each other, but co-operate in such a way that the Word made flesh willed humanly in obedience to his Father all that he had decided divinely with the Father and the Holy Spirit for our salvation (cf. DS 556-559). Christ's human will 'does not resist or oppose but rather submits to his divine and almighty will' (DS 556)" (CCC, 475). This is a fundamental question, since it relates directly to Christ's own being and to our salvation. St Maximus the Confessor was outstanding in his efforts to clarify this doctrine, making very effective use of the well-known passage of Jesus' prayer in the Garden of Olives, which shows the agreement of Christ's human will with the Father's will (cf. Mt 26:39).
A consequence of the duality of natures is also the duality of operations in Christ: the divine operations (or actions) that proceed from his divine nature, and the human operations that proceed from his human nature. We can also speak of theandric operations to refer to those in which the human action serves as an instrument of the divine; this is the case of the miracles worked by Christ.
The reality of Incarnation of the Word was also clarified in the last great Christological controversy of the patristic period: the dispute over images. The custom of representing Christ in frescos, icons, bas-reliefs, etc., is very ancient, going back at least to the second century. The iconoclast crisis in Constantinople at the beginning of the eighth century began with a decree by the Emperor. For centuries, theologians had shown themselves to be for or against the use of images, but both positions had co-existed peacefully. Those who were against images held that God's infinity cannot be enclosed or circumscribed within a limited painting. However, as St John Damascene stressed, the Incarnation itself circumscribed the “incircumscribable" Word. “Since the Word became flesh in assuming a true human nature, Christ's body was finite. Therefore the human face of Jesus can be portrayed (cf. Gal 3:1)" (CCC, 476). At the second ecumenical Council of Nicaea in the year 787, “the Church recognised its representation in holy images to be legitimate" (CCC, 476). Indeed, “the individual particularities of Christ's body express the divine Person of the Son of God. He has made his own the features of his own human body to the extent that, painted on a sacred image, they may be venerated because the believer who venerates his image, venerates the Person it represents." 
Christ's soul, since it was not essentially divine, but human, was perfected, like the souls of the rest of men, by means of habitual grace, which is a “habitual gift, a stable and supernatural disposition that perfects the soul itself to enable it to live with God, to act by his love" (CCC, 2000). Christ is holy, as the archangel Gabriel announced to Mary at the Annunciation (cf. Lk 1:35). Christ's humanity is radically holy, the source and model of the holiness of all men. Through the Incarnation, Christ's human nature was elevated to the greatest unity with the divinity—with the Person of the Word—to which any creature can be raised. From the point of view of Christ's humanity, the hypostatic union is the greatest gift one could receive, and is generally known as the grace of union. Through sanctifying grace, Christ's soul was divinised by the transformation that raises the operations of the soul to the plane of the intimate life of God, giving to its supernatural operations a co-naturalness that it would not otherwise have possessed. His fullness of grace also implies the existence of the infused virtues and the gifts of the Holy Spirit.
From Christ's fullness of grace we all received grace upon grace (Jn 1:16). This grace and these gifts are bestowed upon Christ not only in accordance with his dignity as Son, but also in accordance with his mission as the new Adam and Head of the Church. This why we speak about a “capital" grace in Christ, which is not separate from Christ's personal grace but which highlights his sanctifying action on the members of the Church. For the Church “is the Body of Christ" (CCC, 805), a Body “of which Christ is the Head; she lives from him, in him and for him; he lives with her and in her" (CCC, 807).
The Heart of the Incarnate Word: “Jesus knew and loved us each and all during his life, his agony and his Passion, and gave himself up for each one of us: 'The Son of God… loved me and gave himself for me' ( Gal 20:2). He has loved us all with a human heart" (CCC, 478). Hence the Sacred Heart of Jesus is the perfect symbol of the love with which he continually loves the eternal Father and all men and women (cf. ibid. ).
José Antonio Riestra
Catechism of the Catholic Church, 422-483
Benedict XVI – Joseph Ratzinger, Jesus of Nazareth , London: Bloomsbury Publishing, 2007, pp. 319-355
F. Ocáriz, L. F. Mateo Seco, and J. A. Riestra, The Mystery of Jesus Christ: A Christology and Soteriology Textbook , Dublin: Four Courts Press, 1994
 Cf. Vatican Council II, Const. Lumen Gentium, 9
 Council of Constantinople I, Symbolum, DS 150; cf. Vatican Council II, Const. Lumen Gentium, 55
 Cf. M. Sharkey (ed), International Theological Commission, Texts and Documents 1969-1985 , San Francisco: Ignatius Press, 1989
 Cf. DS 151 and 157-158
 Cf. Congregation for the Doctrine of the Faith, Declaration Mysterium Filii Dei, 21 February 1972, in AAS 64 (1972), 237-241
 Cf. DS 151 and 159
 Cf. DS290-295
 Cf. DS 301; Catechism of the Catholic Church , 467
 Cf. Catechism of the Catholic Church , 467
 Cf. Catechism of the Catholic Church, 423
 Cf. Catechism of the Catholic Church, 425
 Cf. Catechism of the Catholic Church, 429
 Cf. International Theological Commission, 'The Consciousness of Christ Concerning Himself and His Mission' (1985), in Sharkey M. (ed), International Theological Commission, Texts and Documents 1969-1985. (San Francisco: Ignatius Press, 1989)
 Cf. Congregation for the Doctrine of the Faith, Notification, 26 November 2006, no. V
 Council of Nicaea II, DS 601