If Jesus came to die for our sins, why didn't he just die right away? Why did God not ordain that his son would die under Herod's slaying of the infants? If he was the true incarnation of God in human flesh, why not die as our substitute right off the bat? Certainly he would have already been the spotless sacrifice, for who is more sinless than an innocent child?
Well, we know this is not what happened. It happened that Jesus grew up, called disciples, taught, healed. In other words, he lived before he died. So the biblical history being what it is, I have no interest in challenging its sensibility. But, in order to dive into the sense of the story, we may and even must ask why it happened the way it did. Because there must be some reason why Jesus didn't die for our sins right away.
I would guess that the initial answer to this question is that the time between his birth and death was the time of his teaching. In terms of one classic distinction, Jesus first fulfilled his prophetic office before proceeding to fulfill his priestly office. Certainly there is merit in this retort, especially as it brings to light that Christ came to do more than simply die. However, is this answer really adequate? Can Jesus' teaching and atonement be so separated? Does his life have so little to do with his death? Are his teachings just a "meantime" activity? Such an answer does not do justice to either his life or his death.
A more adequate answer to the question may be found by challenging the assumption beneath the question itself. Could Jesus really have died right away? Is the requisite substitute for our sin only God enfleshed in a sinless human being? Isn't there more to our reconciliation with God than this? Yes! The perfection of Jesus cannot be understood as a static freedom from sin. Rather, the perfection of Jesus is accomplished as a perfecting of our human life extended over history. His whole perfected history stands as a substitute for our failed histories - or, better yet, for the failed history of humanity as a whole. Check out the temptation stories (Mt 4 = Lk 4) in contrast to the Deuteronomy stories which Jesus quotes to catch a glimpse of how Jesus succeeded where we failed.
Thinking about the atonement this way not only answers the question, but avoids splitting Jesus' life and death. For Jesus in his entire history - both life and death - is our substitute. He has re-lived (or "recapitulated") the life that we did not live so that we may live again in him. And Jesus as this perfected one (with his "acquired righteousness" so to speak) has died the death we deserved so that we do not have to die.
Does this answer address the concern of the question?
Is the initial answer really as bad as I portrayed it?
How does the resurrection fit in here ... it certainly does - but how?