Christian mythology is powerful because of the idea that God loves you so much he will give you infinite chances to repent and turn away from sin for as long as you live. Carrying guilt and shame for your sins won't fix them, but being sorry for them and working to sin no more will fix your future.

It was even more powerful in an era when the gods were vengeful in their retribution and/or capricious in their favor. I'm not saying I like it or agree with it, but it does really bring some form of comfort to people, which is why it spread so far and lasted for so long.