Scot McKnight has a great post on the Sermon on the Mount. He states that the Sermon on the Mount is the actual summons of Jesus himself for those who want to follow him and not a secondary, advanced ethics for people after they accept Jesus.
Here are a couple of paragraphs from his post:
“Here’s the point for my post today. The SoM has to be understood as Matthew’s presentation of who Jesus is, what he teaches, and what he calls people to be and to do. This is not secondary teaching for the fully committed after they have chosen to “accept” Jesus, but it is what must be understood as the very summons of Jesus itself. In other words, this is Jesus’ evangelistic summons.”
“The SoM is more than the ethics that follow conversion: the SoM is the summons of Jesus himself for those who want to follow him. This is not second-layer stuff but entry stuff; this is not a lesson in ethics but a radical summons to surrender. Indeed, it is ethics and discipleship, but that is because one cannot have conversion or following Jesus without ethics and discipleship.”