
This was a question that initially troubled His early followers - they were traumatised and frightened in the early days after His death, and even though Jesus had appeared to them afterwards, truly alive, it was still something that they struggled to take in. Then they began to re-read the Old Testament, the defining document of their Jewish faith.
In their historic writings and heritage stories, they began to discover that making sacrifices was an important part of putting things right. God had explicitly forbidden human sacrifice, but had invited a practice where animals would be slain, and the shedding of their blood would act as a symbolic punishment for the wrongdoing of sincere worshippers - this was how their mistakes and transgressions would be forgiven. They remembered how Jesus had once been described as "the Lamb who takes away the sins of the world", and began to see that Jesus had become the ultimate sacrifice.
If Jesus was God, then He was infinite - infinitely powerful; infinitely good; infinitely pure and sinless. Whereas an animal might stand as a substitute for a while, Jesus was a sacrifice of infinite proportion who could deal with evil and wrongdoing once and for all. He became the ultimate sacrifice for the whole of humanity. This was how Jesus became "the lamb of God that takes away the sins of the world"