I think I already knew this, but I’m glad Scot McKnight expressed it so well:
Here Jesus rises to the mountain — surely an echo of Moses giving the Torah from Mt Sinai — to reveal God’s will for God’s people of the new creation. The first thing that strikes the reader of this Sermon is its profound christology: the Sermon is Christology on Steroids. The reader (or listener) comes away thinking, “Do I want to give myself to this Jesus?”
In this Sermon, Jesus who reverses all expectations of who is “in” and who is “out,” because Jesus points to people groups who are marginalized and says “The King’s kingdom includes these people.” Jesus says, “No, your way of classifying people is wrong. There’s a new way in my kingdom.” It’s very gospel like for Jesus to assume the posture of the New Moses and to begin classifying people all over again. Then Jesus, in essence, issues his mission: my followers, he says, are to be light and salt, one to the Gentiles and one to the Jews (?) (5:13-16).
Then Jesus utters what has to be one of the most gospel-ish statements in the whole Bible. I quote the words.
Matt. 5:17 “Do not think that I have come to abolish the Law or the Prophets; I have not come to abolish them but to fulfill them. 18 For truly I tell you, until heaven and earth disappear, not the smallest letter, not the least stroke of a pen, will by any means disappear from the Law until everything is accomplished.a 19 Therefore anyone who sets aside one of the least of these commands and teaches others accordingly will be called least in the kingdom of heaven, but whoever practices and teaches these commands will be called great in the kingdom of heaven. 20 For I tell you that unless your righteousness surpasses that of the Pharisees and the teachers of the law, you will certainly not enter the kingdom of heaven.
Now if the gospel is what Paul says it is in 1 Cor 15:3-5, then the gospel is a message about Jesus, a message that claims Jesus fulfills Israel’s hopes (and the divine plans for the cosmos) …. and that is exactly what Jesus does here: Jesus claims in public that he is is the fulfillment of Israel’s Torah and Prophets. That’s gospeling. We have too many diminishing Christ in order to make this stuff Law, or too many who diminish Christ to make this a global vision for global justice. First comes christology, everything else follows. If we get christology right, we get everything else right. Get it wrong, and the whole thing falls apart.
The entirety of Matthew 5:21-48 illustrates that very claim by Jesus: those snippets on vows and lust and divorce and loving your enemies are not morals but Messianic claims on messianic people. Jesus is King and this is how the King’s people live in the King’s kingdom. We dare not delete the king and grab his morals; this only works when we attach ourselves to the King and let the King shape how we live.
I could go on, but will leave that to you. The Sermon on the Mount, folks, is pure gospel because it proclaims Jesus (not just morals and Torah). This is why the Sermon ends with an invitation: take up my yoke, it is saying, and follow me. Jesus sketches his vision for his people and invites us to turn from our current way of life and give ourselves to him and to his kingship.