For about 40 years after the death of Jesus, the early Jesus followers met together for worship and to share their memories about what Jesus said and did. Their memory of who Jesus was, what he had done. But their memory was directly affected by the crucifixion and resurrection of Jesus. They needed an explanation for the crucifixion of Jesus that gave meaning and purpose for that horrible event. Several explanations gradually emerged that drew on their religious practice of sacrifice for forgiveness. If you wanted to be forgiven, you went to the temple and sacrificed an innocent animal. God would see that act, and would respond (according to the temple priests) by forgiving your sins. Gradually the early followers of Jesus began to see Jesus as God's ultimate sacrifice to forgive their sins. Thus in Christian story telling the death of Jesus was seen as the intentional act of God to forgive sins. No human death could do that, because all humanity was sinful and thus they could only die for their own sins. But since Jesus was sinless, his crucifixion death was an effective God act of forgiving the sins of humanity. In more recent years that Substitutionary Atonement explanation for the death of Jesus has been challenged. What does it say about God if God had to have one person killed before God could forgive other people? And if this excruciatingly painful death of Jesus is essential for human salvation, the teachings and life of Jesus loses its meaning (except that Jesus has to be sinless because if he had ever sinned his death would have no salvation impact for others.) In other words, did Jesus see himself as God's sacrificial lamb, or was he a living prophet in the tradition of Isaiah and other Old Testament prophets proclaiming God's intention for all humanity?
