This question would have been more appropriate for Good Friday, but, well, I got lazy [grin].
Anyway, one of the most commonly asked questions for Christians about Christianity surrounds the crucifixtion. Presumably, Jesus died for all of our sins, and as some sort of payment for Original Sin. But there are a few questions about this. This was a sin that we had made against God (presumably) and Jesus is, in fact, also part of God and certainly sent from God. Why is God sending someone to make up for something that we did, and that Jesus didn’t do? If we were the ones who committed the sin, why is our role basically to be ones who commit the next sin and actually kill Jesus? Why would God send his son and have us kill Him before He’ll forgive us? Shouldn’t we be the ones sacrificed or making reparations? And, heck, why couldn’t God just forgive us, instead of going through what looks like a charade? What purpose did Jesus’ death serve?
These are good questions, and I think to start answering them you have to look at a theme that is indeed prevalent in the New Testament from the being: Jesus’ humanity.
The Gospels of Matthew, Mark and Luke all describe the temptation of Jesus in the desert, where He goes off and is tempted by the devil, rejecting the temptations every time. However, in order for this story to have meaning, it has to be the case that Jesus was tempted and really tempted by the temptations. The devil surely would not have tried to tempt Jesus if he knew that Jesus was simply going to be completely unmoved by temptation. And note that the temptations are human ones, and ones that we’d all have to face: the material (you’re hungry, so make yourself some bread), the doubtful (test God to prove God real) and the powerful (I’ll give you the power to rule the world if you change sides). This, to me, is the first indication of the humanity of Jesus, and it is quite early (^ Matthew 4:1-11, New International Version, ^ Mark 1:12-13, NIV, ^ Luke 4:1-13, NIV) in all the editions. This humanity carries on throughout the NT. Jesus gets angry, cursing a fig tree and driving the money changers from the Temple. While a lot of the focus is on teaching, there is an undercurrent of Jesus’ humanity all the way through the NT.
And all of this culminates in the ultimate example of Jesus’ humanity: the Garden of Gethsemane. Jesus knows that He is going to suffer and die. And He prays to avoid it. Theoretically, Jesus himself could avoid it, and it’s clear that He’d really like to avoid it. But He puts it all in God’s hands, and sticks it out to the end, through all the suffering, and even unto death.
So you might be asking at this point “Okay, sure, Jesus’ humanity is important in the NT … but how does this answer the question?”. My thought is that Jesus was not sent to be a sacrifice, but instead to be an example to us. Yes, Jesus — being the Son of God — is better than us, but He is also human. He knows and experiences what we, in fact, experience. He fears what we fear; He feels the pains we feel; He desires what we desire. All of these can, in fact, lead to us sinning; we sin because we are afraid, because we will feel pain if we don’t, and because we want things that are sinful. Being human, as I’ve already said, Jesus knows all that … and He asks us to put being moral — ie not sinning; I’m more a philosopher about morality than a theologian — ahead of those things, just as He did.
And He did it in dramatic fashion. The suffering that He experienced is far more than most of us will experience, and He even went so far as to take the ultimate step, and die for morality. Being Stoic-leaning, of course this idea of morality appeals to me, since it is pretty much in line with what they argue. Ultimately, the call is that you do the right thing, no matter what pain or suffering it brings you, and even if it kills you. And the right thing for Jesus was, in fact, to be that model and demonstrate that principle in an undeniable form.
And, to me, this is what really indicates the split between the OT and the NT. The OT was about following the rules out of fear, and really was about simple obedience. But the OT was a far more brutal world than the world of the NT (even though that world was pretty bad itself). There were many gods, and many fights over gods, and you pretty much had to browbeat people into sticking with your god (see Exodus and the Golden Calf for an example of that.) But the Roman Empire was, in fact, fairly tolerant of other religions and imposed law and order on the conquered nations. Comparatively speaking, the Roman Empire was fairly civilized. And in this case, a shift in morality could occur, from simply following the rules to doing more things just because they were right, with less emphasis on rules and more emphasis on compassion. The Golden Rule is the prime example of the new morality, and Jesus was quite firm on the idea that simply following rules and rituals was not the right way to be religious.
But without the fear of God, a new example needed to be given to ensure that people didn’t just give in to temptation, and Jesus was that example. And He did what was right, and was rewarded for it, and it is promised that we too will be rewarded for it.
Ultimately, Jesus’ death serves to provide us with the example of the new morality, and to cement the demand that while we may not be perfect we should strive for Jesus’ willingness to put even our most extreme desires aside in the service of what is morally right. And that, then, is why Jesus had to die on the cross for our sins.
(Note: Anyone who wants to argue against this on the basis that I need to prove that God or Jesus exists will be summarily ignored; when doing Christian theology and explaining what it means, it is ludicrous to insist that I have to continually preface my statements with “If they’re right”. The question has an implicit “If they’re right” in it, and thus it is entirely fair to have an implicit “If they’re right” in the answer.)