RELIGOUS THREAD (flashing sirens here)

I think most protestant theologians who are honest will have to admit that question does not have a singular, logical answer. They will have their pet theories, and each will be able to support their ideas with scriptural references and reasoning from the old testament, but due to the accepted pluralism of protestant belief, that's as far as they should be willing to go. If someone tells you that they have a definite answer for why god does something, and its not explicitly explained in scriptures, they should immediately be suspect.

The post-modern interpretation Capo gives is definitely one of more popular interpretations taking hold in this day.

A more classical and structural interpretation, and the one you were probably brought up under, is the Calvinist theory of "penal substitution." This starts with the assumption that God's nature as a being of justice compels him, morally and legally, to punish that which does wrong or is sinful. In the pre-Christ era, sheep as sacrifices were given up by humans to take the punishment that should have been meted out on themselves. Jesus, being god, and also perfect, was even more valuable than sheep, and thus was the best sacrifice possible. By this formulation, the "ultimate sacrifice" is not so much one of symbolism, but one of balancing the scales of morality or justice.

The Roman Catholic doctrine of atonement mostly follows this line of reasoning, but also adds that believers must demonstrate that they have been atoned for by exhibiting "condign merit." In other words, you aren't atoned for unless you prove you are such by being good as well.

I am sorry to say I know little about Eastern Orthodox, so I'm not going down that road. However, I do know that their theology is less about morality and justice, and has a more "curative" approach to explaining atonement.

All these explanations are well and good, but they still spawn more questions than they answer. In the end, they all end up admitting that "it just works," without venturing into speculative metaphysics.
Welcome to the wonderful world of trying to rationalize the actions of a god that may or may not even exist in the first place.

Jam it back in, in the dark.