Do not think that I have come to abolish the law or the prophets. I have come not to abolish but to fulfill.The Sermon on the Mount marks the beginning of Jesus' teaching/rabbinic ministry in the Gospel according to Matthew. It was thus appropriate for Jesus, at this phase of his ministry, to affirm the authority of Moses and the prophets on whose shoulders his ministry stood. As a member of the Jewish community, he clearly understood the role played by Moses and the prophets in the life of the community. By making the above statement, therefore, Jesus not only called on the blessing of Moses and the prophets as he began his ministry, but also clearly spelt to the people where he stood. 
Jesus’ coming was a fulfillment of the mission of the prophets who had plied their trade before him. By his life and ministry, not only did Jesus give witness to the validity of the prophecies of old, but he also demonstrated that he was the fulfillment of the Old Testament prophecies. From Moses to John the Baptist, the content of the Old Testament prophecies concerned salvation of God’s people: God coming to dwell with God’s people. The role of the great prophets of old was to reassure the people of God’s presence in their midst especially during those times of turmoil in their land. But as God incarnate, Jesus did more than reassure the people of God’s continued presence. In Jesus Christ, humanity came to behold the face of God. 
Immediately preceding Jesus’ statement quoted above are the beatitudes. The beatitudes come at the beginning of Jesus’ ministry because in them are summarized Jesus’ proclamation of the good news. For unlike the prophets that preceded him, Jesus was not proclaiming something that was to take place sometime in the future. He was proclaiming the good news that he had inaugurated. As a matter of fact, his life and ministry was the very embodiment of his proclamation. As he had made clear at the onset of his ministry, he had come to proclaim a year of the Lord’s favor (cf. Luke 4:18-19). No segment of the society needed the assurance of God’s presence than those who were poor, hungry, mourning, despised, oppressed, and persecuted. It is to these that the prophets of old were sent. It is for the protection of these that the whole corpus of Jewish law was put together. This was later made clear when Jesus laid greater emphasis on the welfare of the people over against the strict observance of the law. What appeared to many as his disregard of the law was actually his commitment to ensuring the law and the prophets were fulfilled.