“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. 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. 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. 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.
There are some within what is called the hyper-grace camp of the church who relegate Jesus’s Sermon on the Mount to his ministry to the Jews and as a function of the Law, simply to condemn them in their sin. Ironically what Jesus actually is doing is talking about a righteousness which can only be realized by grace, and comes from the inside out.
There are aspects of the old covenant in the sermon, such as Jesus’s reference to offering gifts at the altar. But the heart of the sermon is plainly the difference in the righteousness that comes with Jesus and the gospel of God’s grace and kingdom in him. Of course this is fully realized through Jesus’s death and resurrection followed by his ascension and the pouring out of the Holy Spirit at Pentecost. But to relegate the Sermon on the Mount to the old covenant and the Jews, and essentially what we’re delivered from is a misreading of scripture, and a terrible loss.
And for that matter there is much that is rich for us in the Old Testament. God has always had his people, a remnant by grace (Romans 11:1-6). Abraham and David are held up as exemplars for us of God’s grace through their faith (Romans 4). Abraham who God promised to be the father of many nations is called the father of us all by his faith as both an example to us, and the one through whom would come God’s blessing of the Seed who would bless the world, the Messiah, Lord and Savior Jesus (Romans 4; Galatians 3).
The Sermon on the Mount is a centerpiece, perhaps the centerpiece of Jesus’s teaching in the gospel accounts: Matthew, Mark, Luke and John, the Sermon on the Plain in Luke 6 the parallel to it, and Jesus’s Upper Room discourse the eve of his crucifixion also central to his teaching. Of course his teachings are sprinkled all throughout the gospels as in his parables (like the Good Samaritan, and the Lost/Prodigal Son), separate sayings, and his teaching on the destruction of the temple and the end times.
We read to some extent how Jesus’s teaching is fulfilled in the letters which followed after Pentecost. But Jesus’s words stand on their own, as well. To miss them by waving them off as a function of the law is a great loss to the church, not only in terms of losing the teaching, but in the failure to handle accurately the word of truth. Something we all have to keep working at, and hold each other accountable to, in and through Jesus.