This question has spilt some ink. One of the reasons fights over the atonement (what God was doing in Christ on the Cross) are so robust historically is the lack of consensus and plurality of answers from the early church on. Even the Creeds don’t have a theory in them, just that something awesome took place. So when I got this question as part of Christian Piatt’s Banned Questions About Jesus book project I took three different and conflicting answers to youth group. “Jesus forgave people of their sins before he died. How could he do this if he actually had to die in order to save us from sin?I gave three 5 minute appeals to the three different theories after which the youth tore them apart, asked questions, and suggested modifications. In the end they updated and selected the theory I sent in for the book.
One could answer the question by saying that Jesus knew he was going to die and rise so he could forgive with the future known and certain, or possibly that Jesus’ divine identity gave him the ability to forgive sin at will, or one could even suggest that if forgiveness could be given before the cross, then the cross may not have been necessary.
It is important to recognize that in forgiving sins Jesus is acting on behalf of God and was one of the reasons Jesus was opposed by the religious leaders, thus forcing one to explain how Jesus’ identity is tied to that of God. To understand this I have found it helpful to see how Paul re-imagined the sacrificial system in light of Christ’s work.
Traditionally an act of sacrifice began with the sinner transferring their identity to the animal through an act of consecration. Afterward the animal was killed so that the person was reincorporated into the people of God. Paul reverses the process so that the process begins with Christ identifying with us and ends with the consecration, us identifying with that which is sacrificed.
In a sense Paul sees, in Christ, God coming to put an end to sacrifice by turning it upside down and beginning with God’s coming to sinner with Good News. From this perspective it would make sense that Jesus could forgive sin without having died because God had come in Christ to consecrate the world as God’s beloved.
For me what was being sacrificed in Christ is God being God without creation. The activity with the youth was fun and getting share their favorite responses to a stack of questions in Christian’s book was even better. You can check out a couple other author’s answers here and of course get the book here.