The literature covering this text (Matthew 5–7) is extensive, to put it mildly. My concern here is simply how do we understand the essence of this text? Traditionally, Lutherans have looked at it as the fullest statement of Law, showing how the Pharisees had distorted God’s Law by reducing to achievable levels. Thus, Jesus raises the bar of the Law to its full intent. “You have heard it was said [current 1st century teaching]… but I tell you.” Jesus gets to the heart behind the action, addressing anger, lust of the heart, etc.
This approach makes sense especially when we include Matthew 5:20 [Jesus said:] “For I say to you that unless your righteousness surpasses that of the scribes and Pharisees, you will not enter the kingdom of heaven” (NAS). And then the ultimate expression of the demands of the Law in 5:48 “Therefore you are to be perfect, as your heavenly Father is perfect” (NAS). I have used both statements in teaching classes in Catechism and courses on Law and Gospel.
David Scaer in his commentary on Matthew, Discourses in Matthew: Jesus Teaches the Church (CPH, 2004) offers a far different approach. Because he takes the Gospel According to Matthew as a catechetical document, he claims that the catechumen is already prepared for the fulfillment of Christ in all of this. Thus, the Sermon on the Mount is Gospel, because Christ has indeed fulfilled it. He cites 5:17 as evidence of this approach. Scaer writes:
Christology was the key that allowed catechumens to understand the message. To them the Sermon was not a listing of rules but a catalog and fulfillment of promises that Jesus had already accomplished in himself for them. What were later judged to be impossible demands and simple moralisms were in fact descriptions of Jesus. Extract Christology from the Sermon and its message is turned from Gospel into law. (p. 214)
I like the insight and agree with what he has written. But this seems more to be audience-reader driven, and not directly connected with Jesus’ original audience. I would say that until someone comes to faith in Christ, that is, the current state of the Pharisees and others when Jesus originally spoke those words, the Sermon would be Law. Thus, context makes the difference between whether a statement is functioning as Law or Gospel. Even the crucifixion itself can be a demand of the Law (what we do/say/think) or the greatest statement of Gospel (what God does for our salvation).
So, what way do you take the Sermon on the Mount: Law or Gospel?
Regardless, if you have not done so and are interested in a challenging and worthwhile study, consider reading Scaer’s book. You will be blessed by doing so.