Matthew 5:17-20
“Do not think that I came to destroy the Law or the Prophets. I did not come to destroy but to fulfill. For assuredly, I say to you, till heaven and earth pass away, one jot or one tittle will by no means pass from the law till all is fulfilled. Whoever therefore breaks one of the least of these commandments, and teaches men so, shall be called least in the kingdom of heaven; but whoever does and teaches them, he shall be called great in the kingdom of heaven. For I say to you, that unless your righteousness exceeds the righteousness of the scribes and Pharisees, you will by no means enter the kingdom of heaven."
How does Christ fulfill the law? Simple, really. The law demands more from us than we can do. The Children of Israel proved that time after time. But God wants connection with His people. Christ fulfills the requirement of the law to grant us access to God. That was the main point in Him coming.
But why not just do away with it? Because of God's level of holiness it is required in order for us to have real relationship with Him. So the law never really goes away until all either walk in the path Christ brought or are cast into eternal damnation. There are only two ways. Period! Remember, Christ Himself said. I am the way, the truth and the light. No one comes to the Father except through Me.” (John 14:6)
So if we fail in a portion of attaining obedience to the path Christ gave us, we are not necessarily going to hell, but will be found lacking in the place we are called to. Like a person gaining a greater role in their job over someone else because they worked harder. The other person isn't fired, they just don't advance.
Learning the truth of the law will change us. Because this is the Law of Christ. In that we can achieve it, first, through Jesus and not of ourselves. Second by conscience effort to change from who we were born to be to who He has called us to be.
But the greatest part of this is that the Law says when you fail, you risk the wrath of God. But through Christ we have a much different relationship with the Creator. He is Abba Father, like a Daddy not a Master. (Romans 8:15)