"And God so loved the world that he gave his only begotten son, that whosoever believes in him shall not perish but have everlasting life," (John 3:16). So the teaching goes that the world was so screwed up after Adam and Eve ate the fruit from the Tree of Knowledge that something had to be done. Now God went ahead and planted a tree with tempting fruit there knowing full-well what would happen, right? Well if you accept the omniscience of God then, yes. If not, that's a separate argument. Hell, even if you don't accept that God knows the future, it doesn't take a god, or a rocket scientist to realize that if you put fruit in a garden, then put humans who don't know anything about right and wrong (knowledge of which, the fruit gave them) next to it and tell them not to eat it and to obey you, it's unlikely they would listen since they lack the capacity to make moral decisions. In fact, you would naturally assume it was God's fault for not originally endowing humans with rational and moral judgment.
I could go on and on about the logical absurdities in the Expulsion from Paradise myth but the point is that in disobeying God, Adam and Eve gave humanity the stain of sin that would carry on to their children and their childrens' children ad infinitum. This doctrine is the answer to the logical question of why must Christians evangelize? If you would not go to hell for not knowing about Jesus and his teachings then it seems logical that if your goal was to save as many humans from eternal torment in hell, the most efficient way is that you do not tell people at all. Ah, but they'd still rot in hell. Even babies. Because of original sin. Well various churches have differing teachings on this but it seems that original sin is with you when you're born because all humans are evil and can only be saved through the grace of God.
So Yahweh used to accept sacrifices throughout the Old Testament as a method of humanity atoning for its sins. God taught the Israelites when it was appropriate to slaughter animals, usually lambs, and burn them on an altar to him. For the first 4000 years of Hebrew history this was acceptable. I am still unclear how exactly Jesus' death was the key to salvation or how the fact that Jesus "died for me and my sins" makes any sense whatsoever. God apparently required more than burnt offerings after all and decided that in order to justly punish humanity he would have to wipe them out for all of their sins and transgressions, including that original sin we committed in ignorance. But rather than do that, if a worthy sacrifice could be found to replace humanity in general, then everyone could be spared utter annihilation. Since the only sacrifice worthy of God was apparently himself, he finally decided to be incarnated in human form and then made sure that things would work out so that he would be humiliated and crucified by the world's superpower, but in an ironic twist, absolve them from any guilt in doing it. Yes, while Pontius Pilate, a brutal Roman governor, fought for Jesus' life the blood lust of the Jewish crowd was too much. The Jews were the ones who cried for his crucifixion, while the Romans reluctantly complied. After Jesus died, the Roman soldiers who carried out the sentence remarked that "Truly this was the son of God."
So by killing God, God was so satisfied that he washed away the debt of past and future sins. So why do we have to bother living good lives? Well, there's another catch that gets you stuck following Christian rules. God will only look the other way on your sins if you follow the teachings of Jesus (and the Church) and believe that he was and is God. So I guess that is a pretty huge sacrifice to make if only a small percentage of the world even cares enough to accept it. Of course is it really a sacrifice at all if you know you're going to raise yourself from the dead in 2 days? Why was Jesus so scared the night before if he knew? I guess he was just afraid of the pain of dying in anguish, thirsty, sunburned and starving over the course of days. Luckily God died within hours of being up there. He didn't have to suffer the worst part of crucifixion, which was the agonizing slow death over days. The question I couldn't quite get is, why didn't god just spare himself the trouble and forgive our sins from the beginning? He knew we were imperfect beings. He made us to be imperfect beings. So why all the unfair judging?
Maybe God also wanted to go down to Earth and dwell amongst his creation so that he could explain in better detail and through example how he wants us to live. Jesus did make a lot of moral teachings that seem to either refine or contradict orthodox Jewish teachings. But then does that not entail a problem again? Why not simply have explained it better the first time. The whole concept of God having to take a mulligan on his first attempt at Revelation through Moses is a little... shall we say, imperfect? Why again does God chose to reveal information critical to the salvation of the entire world only to a select few people in a tiny insignificant corner of the world, which is itself in a tiny insignificant corner of the galaxy, which is itself a tiny insignificant part of the universe. So vast is this universe that it really makes you wonder why an almighty and omnipotent God's interests are so provincial. I'm getting off topic. So God's revelation was lost in translation through the Israelites. Moses and all the prophets just didn't get it right. So God came and told us how he actually meant to write the Torah.