Someone said ;
"It is not a sin for God to take a life, no more than it is a sin for nature to take a life. If God takes a life he had a good purpose for it. We cannot judge God. We do not have all the facts in order to make that kind of a judgment. We cannot see all ends. God can however, so he is the perfect judge. "
I cannot agree with this. It smacks too much of someone wanting to "have his cake and eat it too". Also, it echos what is found in the Book of Job, which even the author doesn't seem comfortable with (by the discussions of his friend and the sylloques of Job), but only accepts.
Nature is impersonal, and makes no conscious decisions. Everything is arbitrary, though it does follow established patterns and natural law. We would never say that nature intentionally ever killed anyone, just a case of that person boing in the wrong place at the wrong time.
However, when god is personalized, that is, created in the image of man, then god is no longer impersonal. If we impart to god some form of intelligence and forethought, his decisions will no longer appear arbitrary, nor can they be, given the properties we have assigned to it.
The problem with the statement above now becomes more clear. If we further assign to god the idea of a lawgiver, as one who can dictate morality to humans, we now have an even worse moral quandry.
Given this, we can no longer believe in an absolute morality, nor absolute law, or any absolute sense of right and wrong."Absolute" means aboslute, applying to all. For, if god is permitted to live by one set of standards, and humans another, then god's moraliy and laws are not absolute, but relative only.
The logical implication of the above is that of relative morality, and by negation the denial of any absolute standard. Further, I am not the only person to ever see this, the early greek philosophers realized it as well. It would seem that certain of the second temple period Jews (perhaps those from the diaspora, exposed to greek philosophy) realized it as well, and realized that the explanation of Job didn't solve the problem.
Ellion asks a valid question, and the answer is complex. I have only time to outline it here.
First, we must realize that Judaism used its temple, and systems of sacrifice for the atonement of sin or wrongdoing. These sacrifices involved the sacrifice of animals, and the burning of certain fat and organs in offering to their god.(not all sacrifices were for atonement, but many were). But, it is from this, and the events of 70CE that the idea of Jesus sacrificing for all sin for all time a viable concept. If the "wages of sin is death", then it follows some payment can be made in the death of another, in the case of sacrifice, of an animal. (it doesnt make much sense to the modern mind, but given the prevalence of ancient religions with sacrificial rituals, it was accepted in ancient times)
Another idea must be interjected here. There was in Judaism at the time, the legend of a messiah, and him one descended of the royal line of their ancient king David, who was to come and gather together Israel and defeat their enemies (at that time, Rome).
Now, when the Roman invaded Judea in 65-70 CE, they not only destroyed their cities, they destroyed and disassembled their holy temple, the place of sacrifice. Many Jews of the time must have believed that their god would save them from the Romans, because common sense, even of that time, had to have indicated to them that defeating Rome was an impossibility. At the same time, they had recent memory of Judas Macchabeus's defeat of the greeks a couple hundred years earlier, but evne the most common minds realized that the Roman had put the greeks down as well.
The point being, the Jewish revolution against Rome was hopeless from the start, as Flavius Josephus points out.
So, now, we have a people who believed themselves to be favored in Gods eyes, with a system of sascrifice in their temple for atonement, who no longer had a temple, and who had been utterly and totally defeated and humiliated by Rome.(even worse in their eyes, by Gentiles).
So, various surviving groups, especially those outside of Judea, began to think of how to continue their traditions, how to cope with(read as rationalize) this humiliating defeat. Several answers arose from these groups. One was rabbinical Judaism, which represents the survival of Pharisseeism. Another solution was Christianity, which rationalized it thusly. The Jewish messiah (oops, I didnt delve into that, sorry, I added something brief above), who was to save Israel, had in fact already come to the Judeans Jews, and that they had in fact killed him. There were some thematic precedents for this to be found in ancient Jewish tradition, legend and myth.
( What makes this difficult is that Christianity itself stretches and contorts existing Jewish tradition ). As it goes, for killing the messiah, Judea and Israel are punished by god through the actions of Rome.
So, now, we have to rationalize the messiah's mission. It was to save JUdea/Israel, but they killed him. They were hten punished by god through Rome. But, the messiah death was now understood to be as a sacrifice, and no more temple was needed, the sacrifice of the messiah is sufficient for all time to come. Further, that messiah was understood to be god (or some part thereof) himself.
Please understnad that I personally don't believe a word of this. I am only tying to answer your question as to why Christians think Jesus's death atones for sin.
There are many allusions to this in the gospels themselves. The trading of Barabbas for Jesus is understood as parallel to the scapegoat of Leviticus. One is released, one is sacrificed.(Some people understand it more literally, but I've never seen them make their case very well in the light of these midrashic parallels. In Matthew's gospel, we have the most insidious line of all the NT, when Pilot is about to condemn Jesus to death, Matthew says that the Jewish mob utters, "let his blood be upon us and upon our children" (Now that's chilling, and that line is regrettable in light of what it caused in later history). We can also see this as shifting the reponsibility of Jesus's execution from Rome to the Judeans. But, that had been done well up to that point in the story, as least in the stories of the Judean leadership scheming to take Jesus out.
It is an attempt at a reconstruction of some form of Judaism from the ashes of Judea, and not the only reconstruction.
Whole books are written on this subject, and I hope you now have some understanding of why the whole Jesus thing arose from the events of those times.
It is also very difficult to reconstract how much of the NT stories are actual history, and how much are legendary. I believe that ther might very well have been a historical figure who the Jesus of the gospels is based. Perhaps not, perhaps he is an amalgammation of messianic contenders of that time.