In Matthew 4:15-23, Christ began his ministry with the Gospel message to repent for the Kingdom of God is at hand, which was a light to the Gentiles, and the Mosaic Law is how his audience knew what sin is, so repenting from our disobedience to it is a central part of the Gospel message. Furthermore, Christ set a perfect example of how to walk in obedience to the Mosaic Law and we are told to follow his example (1 Peter 2:21-22) and that those who are in Christ are obligated to walk in the same way he walked (1 John 2:6). So Christ spent his ministry teaching his follower to obey the Law of Moses by word and by example and it would be absurd to think that the Law of Christ is something other than or contrary to what Christ taught. God is not in disagreement with Himself about which laws we should follow, so the Law of Christ is the same as the Law of the Spirit and the Law of the Father, which was given to Moses. Moreover, everything taught in the Sermon on the Mount was based on what was taught in the OT, so Christ was not teaching anything brand new in disagreement with what the Father commanded.
