Jesus and His Teaching, the Fulfillment of the Law
(Jesus said to His disciples,)  "Think not that I have come to abolish the law and the prophets; I have come not to abolish them but to fulfill them.  For truly I say to you, till Heaven and earth pass away, not an iota, not a dot, will pass from the law until all is accomplished.  Whoever then relaxes one of the least of these commandments and teaches men so, shall be called least in the Kingdom of Heaven; but he who does them and teaches them shall be called great in the Kingdom of Heaven."
17-19. In this passage Jesus stresses the perennial value of the Old Testament. It is the word of God; because it has a divine authority it deserves total respect. The Old Law enjoined precepts of a moral, legal and liturgical type. Its moral precepts still hold good in the New Testament because they are for the most part specific divine-positive promulgations of the natural law. However, our Lord gives them greater weight and meaning. But the legal and liturgical precepts of the Old Law were laid down by God for a specific stage in salvation history, that is, up to the coming of Christ; Christians are not obliged to observe them (cf. "Summa Theologiae", I-II, q. 108, a. 3 ad 3).
The law promulgated through Moses and explained by the prophets was God's gift to His people, a kind of anticipation of the definitive Law which the Christ or Messiah would lay down. Thus, as the Council of Trent defined, Jesus not only "was given to men as a redeemer in whom they are to trust, but also as a lawgiver whom they are to obey" ("De Iustificatione", can. 21).
Source: "The Navarre Bible: Text and Commentaries". Biblical text taken from the Revised Standard Version and New Vulgate. Commentaries made by members of the Faculty of Theology of the University of Navarre, Spain. Published by Four Courts Press, Kill Lane, Blackrock, Co. Dublin, Ireland. Reprinted with permission from Four Courts Press and Scepter Publishers, the U.S. publisher.