So God and Abraham made this deal, and for a long time it went fine and everyone was happy, except for a few incidents involving Assyrians and Pyramids and locusts and... well, it actually didn't go so well.
And eventually God decided it was the time for the "it's just not working out" talk, possibly to be followed by the "it's not you it's me" segue into the "I hope we can still be friends" line. The problem is, of course, that God, being God, had a hard time communicating with mortals - hence the burning bushes and such. But God felt pretty strongly that this was the kind of discussion that He needed to have in person, so he arranged a way to be mortal enough to walk Earth as a man, sort of.
But once He was here, He started to notice things He hadn't seen from above. He came to love humanity, despite our numerous manifest flaws, and He started to feel like maybe we all could do with another chance or two.
But of course even God has to follow the rules - not because they bind Him, of course, but because God is one of the good guys and the good guys follow the rules (it makes it easy to tell them apart from the bad guys). So humanity was due for some pretty severe discipline, and there really wasn't a way out of it.
But the almost-mortal God (who was going by 'Jesus' because introducing yourself as God tends to meet with polite skepticism in even the most accepting company) decided He could take the punishment for us. And so He did. And, by the look of things, it was none too pleasant. But because Jesus took our medicine for us, we got off the hook.
And that's Christianity in a nutshell. We screwed up, we had a whuppin' coming, and Jesus stepped up and took one for the team.
And even if it's only a story, you can still ask yourself "what would Jesus do?" and it's a useful question.