Gospel (Mt 22:34-40)
But when the Pharisees heard that he had silenced the Sadducees, they came together. And one of them, a lawyer, asked him a question, to test him. “Teacher, which is the great commandment in the Law?”
And he said to him, “You shall love the Lord your God with all your heart, and with all your soul, and with all your mind. This is the great and first commandment. And a second is like it, You shall love your neighbor as yourself. On these two commandments depend all the Law and the Prophets.”
The Pharisees and Sadducees were two very influential groups in the society in which Jesus lived. And they had two very different ways of interpreting the Law. The Sadducees were part of the upper class. From their ranks came, right from the beginning of the Roman occupation, the chief priests who were the Jewish representatives before the imperial power. They were more concerned about politics and the Temple rituals than religious questions connected with daily life. The Pharisees, in turn, were very exact in fulfilling the prescriptions of the Law of God.
Perhaps impressed by Jesus’ marvelous answer to a question raised by some Sadducees, whom he had left speechless, some Pharisees decided to test him with a difficult question. With their meticulous care to fulfil even the smallest indications of the Law, the Pharisees ended up with a list of 613 commandments. Faced with so many different precepts that were hard for anyone to remember, their question was an important one: What is the principle commandment of the Law?
Jesus’ reply surprises them. He doesn’t point to one of the ten commandments in the Decalogue, but rather mentions two that are not specifically part of it. First, he cites a text from the Old Testament that is part of the prayer the Jews called the Shemá, found in the book of Deuteronomy: “Hear, O Israel: The Lord our God is one Lord; and you shall love the Lord your God with all your heart, and with all your soul, and with all your might” (Deut 6:4-5). The second, “you shall love your neighbor as yourself” (Lev 19:18), is one of the many precepts included in what is called the "Code of Holiness" in the book of Leviticus.
What is striking about Jesus’ reply is how he highlights these two commandments that were “lost” amid all the precepts contained in the Law. He mentions them together, thus making clear that love for God and love for neighbor are inseparable and complement one another.
The first is love for God, a love that is correspondence to the One who has loved us first. But what does love for God entail? Benedict XVI explains: “The love-story between God and man consists in the very fact that this communion of will increases in a communion of thought and sentiment, and thus our will and God's will increasingly coincide: God’s will is no longer for me an alien will, something imposed on me from without by the commandments, but it is now my own will, based on the realization that God is in fact more deeply present to me than I am to myself.”
And love for God leads us by the hand, as it were, to love for our neighbor. Benedict XVI continues: “in God and with God, I love even the person whom I do not like or even know. This can only take place on the basis of an intimate encounter with God, an encounter which has become a communion of will, even affecting my feelings. Then I learn to look on this other person not simply with my eyes and my feelings, but from the perspective of Jesus Christ. His friend is my friend. Going beyond exterior appearances, I perceive in others an interior desire for a sign of love, of concern ... Seeing with the eyes of Christ, I can give to others much more than their outward necessities; I can give them the look of love which they crave.”
“If we want to help others, we must love them—I insist—with a love clothed in understanding, dedication, affection and voluntary humility. Then we will understand why our Lord summed up the whole law in that double commandment, which is really just one: love of God, and love of one's neighbor, with all our heart.”
 Benedict XVI, Enc. Deus caritas est, no. 17.
 Ibid., no. 18
 Saint Josemaria, Christ is Passing By, no. 167.