Religion

If you read the Old Testament you'll quickly learn that there were A LOT of "sins" that required you be killed if you committed them.

My theory is the local government called in the church elders and told them, "no more religious killings or we'll ban your church."

And so, they came up with the whole Jesus thing: a sinless man, the Son of God, who died brutally to absorb all the sin of the world, so that all those who believe in Him will be forgiven and go on to eternal life.