From Pope St. John Paul II’s encyclical Veritatis Splendor (1993):
In the “Sermon on the Mount”, the magna charta of Gospel morality,24 Jesus says: “Do not think that I have come to abolish the Law and the Prophets; I have come not to abolish them but to fulfil them” (Mt 5:17). Christ is the key to the Scriptures: “You search the Scriptures…; and it is they that bear witness to me” (Jn 5:39). Christ is the centre of the economy of salvation, the recapitulation of the Old and New Testaments, of the promises of the Law and of their fulfilment in the Gospel; he is the living and eternal link between the Old and the New Covenants. Commenting on Paul’s statement that “Christ is the end of the law” (Rom 10:4), Saint Ambrose writes: “end not in the sense of a deficiency, but in the sense of the fullness of the Law: a fullness which is achieved in Christ (plenitudo legis in Christo est), since he came not to abolish the Law but to bring it to fulfilment. In the same way that there is an Old Testament, but all truth is in the New Testament, so it is for the Law: what was given through Moses is a figure of the true law. Therefore, the Mosaic Law is an image of the truth”.25
Jesus brings God’s commandments to fulfilment, particularly the commandment of love of neighbour, by interiorizing their demands and by bringing out their fullest meaning. Love of neighbour springs from a loving heart which, precisely because it loves, is ready to live out the loftiest challenges. Jesus shows that the commandments must not be understood as a minimum limit not to be gone beyond, but rather as a path involving a moral and spiritual journey towards perfection, at the heart of which is love (cf. Col 3:14). Thus the commandment “You shall not murder” becomes a call to an attentive love which protects and promotes the life of one’s neighbour. The precept prohibiting adultery becomes an invitation to a pure way of looking at others, capable of respecting the spousal meaning of the body: “You have heard that it was said to the men of old, ‘You shall not kill; and whoever kills shall be liable to judgment’. But I say to you that every one who is angry with his brother shall be liable to judgment… You have heard that it was said, ‘You shall not commit adultery’. But I say to you that every one who looks at a woman lustfully has already committed adultery with her in his heart” (Mt 5:21-22, 27-28). Jesus himself is the living “fulfilment” of the Law inasmuch as he fulfils its authentic meaning by the total gift of himself: he himself becomes a living and personal Law, who invites people to follow him; through the Spirit, he gives the grace to share his own life and love and provides the strength to bear witness to that love in personal choices and actions (cf. Jn 13:34-35).
24 Cf. Saint Augustine, De Sermone Domini in Monte, I, 1, 1: Corpus Christianorum, Latin series, 35,1-2.
25 In Psalmum CXVIII Expositio, Sermo 18, 37: PL 15, 1541; cf. Saint Chromatius of Aquileia, Tractarus in Matthaeum, XX, I,1-4: Corpus Christianorum, Latin series, 9/A, 291-292.
— Veritatis Splendor 15