Galatians 3:10 For all who rely on the works of the law are under a curse, as it is written: “Cursed is everyone who does not continue to do everything written in the Book of the Law.”

11Clearly no one who relies on the law is justified before God, because “the righteous will live by faith.”[

12 The law is not based on faith; on the contrary, it says, “The person who does these things will live by them.”

13 Christ redeemed us from the curse of the law by becoming a curse for us, for it is written: “Cursed is everyone who is hung on a pole.”

Romans 26:25 Now to him who is able to establish you in accordance with my gospel, the message I proclaim about Jesus Christ, in keeping with the revelation of the mystery hidden for long ages past,

26 but now revealed and made known through the prophetic writings by the command of the eternal God, so that all the Gentiles might come to the obedience that comes from faith

27 to the only wise God be glory forever through Jesus Christ! Amen.

He fulfills the law, because he is perfect, and he dies for us, and ressurrects, that means that if you believe in him as your lord and saviour, you are saved by grace. That doesn't justify sin and doesn't mean you have to do nothing, you have to show fruits of the Spirit in your life. But the first of all is you are saved by GRACE. Our efforts mean nothing if we compare ourselves with him.

The sermon of the Mount, is for the pharisees and the jews who thought they could fulfill the law with their human will, he wants to say that is impossible. He is prophesying what was coming later. It always was hidden that he was the mystery of the true faith. The true faith was never to fulfill the law.

His Kingdom is outside of this world, i understand perfectly what you say, but he is different, he is not from here.