Matthew 5:17-19 (Wednesday's gospel)
17 "Do not think that I have come to abolish the law or the prophets. I have come not to abolish but to fulfill.
18 Amen, I say to you, until heaven and earth pass away, not the smallest letter or the smallest part of a letter will pass from the law, until all things have taken place.
19 Therefore, whoever breaks one of the least of these commandments and teaches others to do so will be called least in the kingdom of heaven. But whoever obeys and teaches these commandments will be called greatest in the kingdom of heaven.
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The subject of Christ fulfilling the law can be tough to get your mind around. I know, I've tried. In very basic terms though, here is my 25-cent version: Christ is the only person who could perfectly live the Mosaic Law, and he did it for all of us. This immediately raises a huge stumbling block though. If Christ fulfilled the Law, how does it apply to us today? Are we bound to try to fulfill it?
Immediately after this passage, through the end of Matthew 5, Christ gives divine interpretation of some elements of the Mosaic Law. Clearly the moral teachings still stand, and he explained them in further detail. To get a flavor, take some time and read through the Matthew 5, 6, and 7 (ie, the Sermon on the Mount).
For anyone who wants more … here's what the Catechism has to say.
I. JESUS AND THE LAW
577 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. For truly I tell you, until heaven and earth pass away, not one letter, not one stroke of a letter, will pass from the law, until all is accomplished. Therefore, whoever breaks one of the least of these commandments, and teaches others to do the same, will be called least in the kingdom of heaven; but whoever does them and teaches them will be called great in the kingdom of heaven. 329
578 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". 330 He is in fact the only one who could keep it perfectly. 331 On their own admission the Jews were never able to observe the Law in its entirety without violating the least of its precepts. 332 This is why every year on the Day of Atonement the children of Israel ask God's forgiveness for their transgressions of the Law. The Law indeed makes up one inseparable whole, and St. James recalls, "Whoever keeps the whole law but fails in one point has become guilty of all of it." 333
579 This principle of integral observance of the Law not only in letter but in spirit was dear to the Pharisees. By giving Israel this principle they had led many Jews of Jesus' time to an extreme religious zeal. 334 This zeal, were it not to lapse into "hypocritical" casuistry, 335 could only prepare the People for the unprecedented intervention of God through the perfect fulfilment of the Law by the only Righteous One in place of all sinners. 336
580 The perfect fulfilment of the Law could be the work of none but the divine legislator, born subject to the Law in the person of the Son. 337 In Jesus, the Law no longer appears engraved on tables of stone but "upon the heart" of the Servant who becomes "a covenant to the people", because he will "faithfully bring forth justice". 338 Jesus fulfils the Law to the point of taking upon himself "the curse of the Law" incurred by those who do not "abide by the things written in the book of the Law, and do them", for his death took place to redeem them "from the transgressions under the first covenant". 339
581 The Jewish people and their spiritual leaders viewed Jesus as a rabbi. 340 He often argued within the framework of rabbinical interpretation of the Law. 341 Yet Jesus could not help but offend the teachers of the Law, for he was not content to propose his interpretation alongside theirs but taught the people "as one who had authority, and not as their scribes". 342 In Jesus, 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. 343 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. . ." 344 With this same divine authority, he disavowed certain human traditions of the Pharisees that were "making void the word of God". 345
582 Going even further, Jesus perfects the dietary law, so important in Jewish daily life, by revealing its pedagogical meaning through a divine interpretation: "Whatever goes into a man from outside cannot defile him. . . (Thus he declared all foods clean.). . . What comes out of a man is what defiles a man. For from within, out of the heart of man, come evil thoughts. . ." 346 In presenting with divine authority the definitive interpretation of the Law, Jesus found himself confronted by certain teachers of the Law who did not accept his interpretation of the Law, guaranteed though it was by the divine signs that accompanied it. 347 This was the case especially with the sabbath laws, for he recalls, often with rabbinical arguments, that the sabbath rest is not violated by serving God and neighbour, 348 which his own healings did.
