Galatians 4:1-5  :

But I say that so long as the heir is a child, he is no different from a bondservant, though he is lord of all; but is under guardians and stewards until the day appointed by the father. So we also, when we were children, were held in bondage under the elemental principles of the world. But when the fullness of the time came, God sent out his Son, born to a woman, born under the law, that he might redeem those who were under the law, that we might receive the adoption of children.

The law is not really our parent. It is more like a guardian, and it is only a temporary one. The law held us prisoner even though we were to become God’s children. We were in a prison, but it was one that was for our own good until the time that Jesus would be revealed. It was for our own good because it taught us to recognize our sin for what it was.

Not many these days have actually tried to obey Moses’ law, but we have tried to obey some of them whether we knew that God also wrote them down or not. God wrote down that we should not steal. Most of us were told by our parents or by the cultures that we live in not to steal. Many of us have stolen anyway. The law of our culture was good at showing us that we were thieves. The same goes for lying. Many of us were told by our parents not to lie. We did anyway, so the law in that case made it clear that we were liars. The law was good at making us ready to realize that we needed to be changed so that we wouldn’t be thieves and liars anymore.

Now, Jesus was born under the strict Mosaic law and He outperformed everyone else. Jesus didn’t need any sacrifice for His own sin. God took this perfect one and put our sin on Him when He died. That paid our sin’s punishment and made us true children of God. This change was internal and by faith we would be able to actually live good lives.

In order to do this transaction, Jesus had to be born as a human and had to live without sin and finally had to die. That is how we were saved. Our own working of laws does not work. God knew that this would be happening in the future during the time the law was in charge. But when the event actually took place, the temporary system was not required to keep people in check. Faith had come and the promise was applied.