Thank you for the interpretative question, “What’s the main idea behind the “sermon of the mount”?
The main idea for the “Sermon on the Mount” is that kingdom people are to live according to kingdom principles.
John the Baptist and Jesus with his disciples preached that the kingdom of God is near. The Jewish people who believed that message were baptized in the Jordan River. John called for a repentance and a changed lifestyle. Jesus elaborated on how a truly repentant believer ought to live in anticipation of the kingdom of God. It is not according to the letter of the Law or according to the tradition of the teachers of the Law but according to the spirit of the Lord. Jesus emphasizes that it is not enough just to hear the message of repentance but the doers of the kingdom principle will find that one’s life is able to withstand the storms of life. That person has laid up treasures in heaven and nothing in this world can affect it while those who laid up treasures on earth will experience a loss when trials come or is stolen by others.
Thus for readers, we must ask ourselves, “What and where is our treasure? Is it on earth or is it in heaven? We know that we can’t take anything physical possession with us into heaven Thus we ought not to grasp so tightly our earthly treasures but rather use those treasures to invest into the kingdom of God. That is the wise builder who builds his life on kingdom principles.
SUMMARY: The Sermon on the Mount are principles for repentant, redeemed believers to live their lives in anticipation of the coming kingdom of God. Their light is to shine so that it glorifies the Heavenly Father and attracts others to the kingdom.
For more perspectives:
https://www.quora.com/Whats-the-main-idea-behind-the-sermon-of-the-mount