I consider this the hardest section to interpret of the Sermon on the Mount, but I can't very well skip it, so I'll give it my best shot.
You start out thinking, "Okay, I understand Jesus saying that he has come to fulfill the Law and the Prophets." The entire Old Testament did predict his coming. You remember how Jesus appeared to his disciples on the Road to Emmaus and "beginning with Moses and with all the prophets, he explained to them the things concerning himself in all the Scriptures" (Luke 24:27). Then Jesus goes on to say in our passage, "Until heaven and earth pass away, not the smallest letter or stroke shall pass away from the Law, until all is accomplished." Fine. I can see how even every "letter or stroke" of the Law is accomplished, whether Jesus is fulfilling the Law's demands for obedience and sacrifice, or fulfilling the promises and predictions spoken by the prophets.
I run into problems with the next statement: "Whoever then annuls one of the least of these commandments, and so teaches others, shall be called least in the kingdom of heaven; but whoever keeps and teaches them, he shall be called great in the kingdom of heaven." Now Jesus seems to be laying the burden of keeping "the least of these commandments" upon his followers, too. But that seems like an impossible task. Furthermore, he goes on to say in his next breath, "For I say to you, that unless your righteousness surpasses that of the scribes and Pharisees, you shall not enter the kingdom of heaven."
Surpassing the righteousness of the scribes and Pharisees makes me automatically think I have to be more pedantic, more anal, and more enslaved to the Law than they. I imagine I have to study the Law with a more powerful magnifying glass and check off a longer "to do" list. And yet, when I recall how Jesus interacts with the Pharisees he typically accuses them of straining at gnats while ignoring weightier matters of the law such as mercy and compassion. He never says, "Aha! See this small detail you missed? Commandment number 217 to be exact."
His teaching instead agrees with what Paul said: "Love does no wrong to a neighbor; love therefore is the fulfillment of the Law" (Romans 13:10). The law of love, mercy and compassion must be the righteousness that Jesus is talking about, which surpasses the Pharisees' righteousness and sums up all the commandments. This gets back what he was saying earlier about the blessed ones: being poor in spirit, meek, merciful, mournful, peaceable. It's not just an external, mechanical obedience, but an obedience to God from the heart. Jeremiah put it this way: "I will put my law within them, and on their heart I will write it; and I will be their God and they shall be My people" (31:33). Paul later writes this: "You are a letter of Christ, cared for by us, written not with ink but with the Spirit of the living God, not on tablets of stone but on tablets of human hearts" (2 Cor. 3:3).
It all makes sense when you consider that this teaching will lay the foundation for the rest of Jesus' sermon. He's going to be talking about doing more than just avoiding murder, but not hating your brother in your heart. He will say that you can't just restrain yourself from committing adultery, but you shouldn't even look at a woman lustfully in your heart. We're talking about a righteousness that pierces straight to the heart.
And yet there is one thing that still bothers me. We started out recognizing that Jesus is the one who fulfills the Law and the Prophets, but Jesus has made the seamless transition into saying that we too should keep and teach these commandments and live by a higher righteousness, as if there can somehow be a comparison between the way he fulfills the Law and the Prophets and the way we do. I just don't see how there could be a comparison. In my Christian experience I definitely know that the Spirit leads me on a path of love and mercy and compassion that is not just a legal and external righteousness. However, I know that only Jesus has lived that life perfectly. He's the one that has actually walked this path, whereas my life is just a pale and ghostly reflection of his. His righteousness is solid and real and has merited God's favor. Mine is this "just do your best" deal that is all raggy with holes in it.
In other words, when Jesus says, "Live this new way of righteousness, a righteousness that comes from the heart" I find myself saying both, "Yes, I recognize myself in this teaching; I do know that higher way of living," and "No, I could never fulfill that demand because of my remaining inner corruption. Only Jesus can save me." So is Jesus making a serious demand of me? Or does he just want to appear to be a serious so he can lead me on a path that is really intended to show me my inadequacy? I have a hard time believing that Jesus would be that disingenuous throughout his entire sermon. That doesn't sound like him. Anyhow, this is a tension that's probably going to be in the back of my mind as we go through the rest of the Sermon on the Mount.