"You are the light of the world.  A city built on a hill cannot be hid.  No one after lighting a lamp puts it under the bushel basket, but on the lampstand, and it gives light to all in the house.  In the same way, let your light shine before others, so that they may see your good works and give glory to your Father in heaven."

Christ is the perfect fulfillment of the Old Testament law and prophets.  The law is the Pentateuch, the Book of Moses, or simply - the first five books of the Old Testament.  We find therein the special Election of the people of Israel, God's chosen people out of whom his Son would come; their Redemption from slavery in Egypt, foreshadowing Christ redeeming the world from sin; the Covenants God made with them from Noah to Abraham to Moses, which would culminate fully and perfectly in Christ; the Law given to them, notably in the Ten Commandments, which prepared mankind to receive the incarnation of the Law in the person of the Christ, who is Law himself and who interiorized its content while keeping the full force of the moral code.

Christ was also foreshadowed through all the Prophets, their warnings and admonitions.  He was foreshadowed through Israel's Wisdom literature, which speaks of Wisdom so deeply and powerfully it goes so far as to personify it and say "she was present to God from the beginning and through her all things were made."

Christ then is the key to understanding all of salvation history: the Election, the Redemption, the Covenants, the Law, the Judges, the Kings, the Prophets and Holy Wisdom.  He tells us this here during the Sermon on the Mount, he shows us later during his dazzling Transfiguration, during which Moses and Elijah, representing the Law and the Prophets, appear and converse with him. (Matthew 17.1-3)  "The scriptures testify on my behalf," he says.  "If you believed Moses, you would believe me, for he wrote about me." (John 5.39, 46)

"Not one stroke of a letter will pass from the law until all is accomplished."  Why such insistence?  Because during this sermon Christ is about to break the law wide open and show how it proceeds entirely from the heart and not just stone tablets and he does not want us imagining he is destroying the law.  "You have heard it said 'You shall not commit adultery.'  What I say to you is whoever looks lustfully at a woman has already committed adultery with her in his heart" and so on.  He does not abolish the law, he perfects it.  The law is just as binding as before but now it is moved entirely inside us because it is there that Christ wishes to establish his kingdom.

"When Gentiles, who do not possess the law, do instinctively what the law requires, these, though not having the law, are a law to themselves.  They show that what the law requires is written on their hearts, to which their own conscience also bears witness." (Romans 2.14-15)