Jesus came to reform, not reject, his tradition. Verse 5:18 shows that he shared the view of the first-century Jews that scripture was inspired by God and could not be set aside. Yet his high view of scripture did not prevent his innovative interpretations that departed from the strict letter of the law. At the same time he affirms the authority of Scripture, he affirms his role as its authoritative interpreter, in keeping with the ultimate will of God. Faithfulness to his interpretation leads to a righteousness that exceeds that of the scribes and Pharisees, not in quantity but in quality. To read the Sermon on the Mount as an impossible possibility is to forget who it is who is preaching. This is the one anointed by the Spirit, sent by the God of Abraham, Moses, David, and the prophets to speak on God's behalf.
The Sermon on the Mount is not an impossible impossibility. It is not, in the end, just about what we are to do. It is about what God is doing. In his book The God of Jesus: The Historical Jesus and the Search for Meaning, Stephen Patterson concludes that the picture of the God of Jesus in the synoptic gospels is of a God "who demands all but also gives all." In service to that God, the Sermon on the Mount becomes a moral vision for the life of the community to which each of us contributes.
The Sermon on the Mount is for everyone, not just a spiritual elite few. We are to read it and move toward being the salt and the light, not just realize our human inadequacy in comparison with the holiness of God. We are to read it as a way of living in keeping with God's moral vision for humankind made possible by the demanding, but also forgiving and empowering presence of God in Jesus.
My skit made fun of me for presuming to critique the preaching of Jesus. In retrospect, my silly skit contained a grain of truth. When we read the Sermon on Mount or a portion of it out of context of the gospel of Matthew, all that is left for us to do is criticize it for being extreme or criticize ourselves for being inadequate. When we read it out of context from who is speaking it, we forget that the one speaking is the new Moses, the anointed Son of God. It turns out that Matthew 5:20 ("Unless your righteousness exceeds that of the scribes and Pharisees, you will never enter the kingdom of heaven") is not a negative ending. It is a positive beginning of a life of following Jesus out of the classroom and into the waiting world.
Dale C. Allison, The Sermon on the Mount: Inspiring the Moral Imagination, The Crossroad Publishing Company, 1999
Alyce M. McKenzie, Matthew in the Interpretation Bible Study Series, Westminster John Knox Press, 2002
Stephen J. Patterson, The God of Jesus: The Historical Jesus and the Search for Meaning, Trinity Press International, 1998