Read Matthew 5:17-20 and Deuteronomy 18:15
In case you have not seen Jesus clearly in our study of The Law, The Prophets, and The Writings, this passage leaves no doubt. Jesus says in Matthew 5:17-18 that he came to fulfill the whole Old Testament. All of the Old Testament poetry, history, and prophecy make clear that the coming Messiah would be a Prophet, Priest, and King. Jesus perfectly fulfills those roles, however, the way he does so may disappoint or even offend you.
The Jewish religious leadership had conceived a notion of what the Kingdom of God would look like when the Messiah came to set it up. Their legalistic initiatives were not so much to earn their entrance to heaven as they were to prepare for the arrival of the Kingdom and to put themselves in advantageous positions when it came. What offended them about Jesus was that he announced that with him the Kingdom had come and he disappointed their expectations.
Some misunderstand this passage, thinking that it only refers to what Jesus came to do for those who trust him, namely, supply the righteousness that cannot be found in the law. Certainly, that was a large part of what he came to do. But here Jesus mentions not only the Law, but also the Prophets. That was a short-hand way of referring to all of the Old Testament writings. Elsewhere, Jesus made it clear that he had to fulfill all of the roles anticipated by the Old Testament: prophet, priest, and king (Mt. 26:56; Lk. 18:31; 24:25-27,44). Jesus' fulfillment of these offices is supremely loving toward those he is redeeming and we must embrace him for it.
One of the largest roles he came to fulfill was that of Prophet. Every time you see the positive function of a prophet, priest, or king in the Old Testament you are encouraged to see the anticipation of Christ. The Catechism explains that "Christ executes the office of a prophet by revealing to us by his Word and Spirit the will of God for our salvation."
We must understand that salvation does not refer simply to your initial conversion but to the whole progress of your redemption from conversion to glorification. Therefore, Christ as a prophet teaches us everything we need to know to complete that process of salvation. In John 14, Jesus says, "If it were not so, I would have told you." And we have noticed numerous times that Paul says, "I do not want you to be ignorant." Those phrases express Christ the Prophet's role of teaching us everything we need to know for salvation.
Jesus' fulfillment of these offices is supremely loving toward those he is redeeming and we must embrace him for it.
You might wonder how that ministry could possibly be disappointing. However, it can be when Christ tells you something you do not want to hear. He reveals everything you need to know whether it comforts you, challenges you, rebukes you, causes you sacrifice, or scares you. That is exactly the kind of prophet the Old Testament prophesied. Moses told the people that God would raise up a prophet just like himself and they must listen to him (Dt. 18:15). Moses was not always popular with the people—in fact at times they wanted to stone him—because he told them what they needed to hear, not just what they wanted to hear.
Think of what kinds of things upset the Jewish religious leaders and see if they do not at times rub you the wrong way to.
Love universally: He commanded them to love people they hated like Gentiles and Samaritans. You too are commanded to love those who hate you, do good to those who persecute you, and serve those who are different from you.
Love sincerely: He exposed their hypocrisies and the real nature of their sins. Likewise, he will not allow you to call your wandering affections anything but adultery, your selfishness anything but idolatry, and your failure to keep commitments anything but taking the Lord's name in vain.
Love selflessly: He chastened them for their lack of compassion for the poor and likewise tells us we must find a way to serve them. He confronted their lack of concern for the unsaved and likewise commands us to give away our faith. He confronted their failure to stand up for the weak and likewise commands that we find a way to defend the unborn, the unwanted, and the handicapped.
However, he tells you these things as one who can be trusted and one who loves you more than anyone else. Therefore, responding to him with a righteousness that exceeds the Pharisees' is to listen to him with love. This too was anticipated in Moses. It was true that no matter how difficult his words were, they came from one who could be trusted implicitly and only spoke them out of love. The same Moses who stood against the idolatries of his people was the same one who insisted that God blot him out of his book before he turn his back on his people.
No matter how hard Christ's words may be or how offensive they sound at first, you may know he can be trusted and he only speaks out of love. As your Prophet, he will never lead you astray or fail to love you no matter how difficult his words may be to hear at times.