Oswald Chambers sums up what the Sermon on the Mount is all about.

“He came to make us what He teaches we should be.”

The teaching of the Sermon on the Mount produces despair in the natural man–the very thing Jesus meant it to do. As long as we have a self-righteous, conceited notion that we can carry out Our Lord’s teaching, God will allow us to go on until we break our ignorance over some obstacle, then we are willing to come to Him as paupers and receive from Him. ‘Blessed are the paupers in spirit,’ that is the first principle in the Kingdom of God. The bedrock in Jesus Christ’s kingdom is poverty, not possession; not decisions for Jesus Christ, but a sense of absolute futility–I cannot begin to do it. Then Jesus says–Blessed are you. That is the entrance, and it does take us a long while to believe we are poor!

Matthew 5:3