The Beatitudes (Matt.5:1-12) not only introduce the Sermon on the Mount but provide the “text” of Jesus’ famous sermon. In other words, what he would say in the rest of the Sermon on the Mount (Matt.5-7) is a filling out of the Beatitudes. They also point to the true meaning of the “kingdom” – not something that is visible, physical or political but that which is in our hearts (Luke 17:21).
Salvation does not come however by living by the Sermon on the Mount. It is interesting but sad that so many non-believers are fascinated by the Sermon on the Mount and hail Jesus as a wonderful teacher because of its content. What they don’t grasp is that the language of the Sermon on the Mount is Jesus’ doctrine of the Holy Spirit. Not only that; we cannot begin to keep the Sermon on the Mount until we have been saved – that is, converted by the Holy Spirit. We are saved not by keeping the Sermon on the Mount but by transferring the faith that we have in our good works to what Jesus has done for us on the cross. When Jesus said that he had not come to destroy the Law but to fulfill it (Matt.5:17) – the most “stupendous” claim Jesus ever made (according to Dr. Martyn Lloyd-Jones), he was pointing to the way we are saved. Not by our keeping the Law or affirming Jesus’ interpretation of it but by recognizing that Jesus kept the Law for us.
When Jesus said he had come to fulfill the Law it was a daring announcement early on in his ministry that he himself – and by himself – would literally keep the Mosaic Law. Not merely the Ten Commandments (which nobody had ever done) but also the finer points of the Law (over 2,000 pieces of legislation). Why did he say he would do this? It is because the Law needed to be fulfilled. No body had fulfilled it. Ever. All have sinned and come short of the glory of God. Even those Jews who were the most candid but loathe to let Gentiles in on the Covenant admitted that no one had kept the Law (Acts 15:10). So when Jesus said he would keep the Law it was an amazing commitment
But the mission was accomplished when he died on the cross and “finished” the work he was sent to do (John 19:30). We are therefore saved by trusting what Jesus did for us: he fulfilled the Law by his sinless life and sacrificial death.
That said, what is the point of his teaching? The answer: once we are saved by faith in Jesus’ blood we are called to persist in faith (Col.2:6-7). Persisting in faith leads us to apply Jesus’ teaching – starting with the Beatitudes. When for example we are broken, meek, pure in heart and hunger for righteousness we inherit the kingdom. Kingdom living is what believers are required to do in order to show they have been saved and that they take Jesus’ teaching seriously.
Read more on in depth analysis in RT’s book.