In earlier times, a spotless lamb without defect was required for sin offerings. Later, Jesus, the Lamb of God, came into the world as a man to walk among us. He was later crucified and killed for the sins of many so that we may have salvation through the sacrifice of God's only son. This fulfilled God's will.
What's not often talked about is the time in between sacrificing animals and sacrificing Christ. The sacrifice of animals began to turn into the sacrifice of God's children. I believe it is not often spoken about because it was not a physical sacrifice that many can perceive with their senses. It was a spiritual sacrifice that was beginning to happen through labeling people "clean" or "unclean", and it still happens to this day in religious gatherings.
Did they not also make these observations for their sin offerings? Yes, as God directed, but the minds of men began to prevail because they leaned on their own understanding. Six hundred thirteen laws to obey that caused oppression rather than liberation. That's why God kept it simple with the ten commandments when instructing the Israelites after the exodus. But when his commandments were turned into a burden he sent his Son, Jesus the Messiah. Jesus said, "I require mercy not sacrifice". The final sacrifice to cover the sins of many, that his people would receive salvation through his Son and stop the sacrifices of his children. He did this to instruct us in his way, the way that leads to mercy.
