The fulfilment of the Law
At the beginning of the Sermon on the Mount Jesus issued a solemn warning in which he presented God's law, given on Sinai during the first covenant, in light of the grace of the New Covenant: “Do not think that I have come to abolish the law or the prophets: I have come not to abolish but to fulfil.”...
Jesus, Israel's Messiah and therefore the greatest in the kingdom of heaven, was to fulfil the Law by keeping it in its all embracing detail - according to his own words, down to "the least of these commandments". He is in fact the only one who could keep it perfectly... The perfect fulfilment of the Law could be the work of none but the divine legislator, born subject to the Law (Ga 3:10) in the person of the Son. In Jesus, the Law no longer appears engraved on tables of stone but "upon the heart" ( Jr 31:33) of the Servant who becomes "a covenant to the people" (Is 42:6), because he will "faithfully bring forth justice" (Is 42:3). Jesus fulfils the Law to the point of taking upon himself "the curse of the Law" (Ga 3:13) incurred by those who do not "abide by the things written in the book of the Law, and do them" (Ga 3:10), for his death took place to redeem them "from the transgressions under the first covenant" (Heb 9:15)...
Jesus taught the people "as one who had authority, and not as their scribes" (Mt 7:29). In him, the same Word of God that had resounded on Mount Sinai to give the written Law to Moses, made itself heard anew on the Mount of the Beatitudes. Jesus did not abolish the Law but fulfilled it by giving its ultimate interpretation in a divine way: "You have heard that it was said to the men of old. . . But I say to you. . ." (Mt 5:33-34). With this same divine authority, he disavowed certain human traditions of the Pharisees that were "making void the word of God" (Mc 7:8.13).
St. Ignatius of Loyola