If you want to understand the theology of Jesus (what he believed about God, about people, about right and wrong, about human responsibilities, etc.) then the Sermon on the Mount (chapters 5-7 of Matthew) is one of the most important sections in the New Testament. It is also one of the best loved and most often quoted. It's not really a sermon, but it has more wisdom, challenge, and depth than any single sermon could. Because it is so packed with meaning, I'm going to take this section slowly.
I say this isn't a sermon because it's not structured like one. A good sermon (like any other kind of good speech) should focus on a single, memorable point. As wonderful as the Sermon on the Mount is, it is much too complicated for that.
In their excellent study, First Light, theologians Marcus Borg and John Dominic Crossan discuss how Jesus taught. Rather than giving them a set of rules or doctrines to memorize, he would use his parables as the start of a discussion and make them think it through for themselves. It’s a great method of teaching, but one that relies on the teacher being there. When you’re writing a book, you lose the back and forth quality, so the author of Matthew did the next best thing. He gathered together a lot of the most important teachings of Jesus and arranged them in logical order. The author of Luke did something very similar on a smaller scale in what’s known as the Sermon on the Plain. It’s different than the Sermon on the Mount in several important ways, but for now we'll just look at the setting.
Why did the author of Matthew put this collection of teachings on a mountain while the author of Luke put them on a plain? Well, in Matthew's case, the author wanted to make his audience think of something very familiar. Remember, we believe this Gospel was written for a predominantly Jewish community living away from Judea (possibly in Sirya.) The author wanted his audience to see that the story of Jesus was consistent with God's actions in the Hebrew Scriptures. By having Jesus speak from a mountain, he is reminding them of Moses, bringing the law from Mount Sinai. He's setting up Jesus as a new Moses and his teachings as a new law.
This was a pretty radical approach, and so the author is quick to point out this doesn't mean that Jesus is getting rid of Moses or the ancient laws. In Matthew 5:7-10 Jesus says that he hasn't come to do away with the old scriptures but to complete or fulfill them. He follows that up by talking about how important the Law and the Prophets are.
I've run across some people (usually on the fringes) who insist that Jesus is telling us that we have to follow every single law of the Hebrew Scriptures scrupulously or else God will send us to Hell. I'll get into exactly how wrong that reading is later. For now, I'll just say that fulfilling the law, in Jesus' teaching is much more about being a loving and faithful person, devoted to justice and mercy, than it does with being a meticulous rule follower.
More on that later. Next up: The Beatitudes!