Atheists simply follow it because it's best for them, not because God is threatening them lest they don't. Indeed, I often find atheists motives for doing good more pure than many Christians, who do it only because they are afraid of divine retribution.
ARRGGGGHH.
ALL OF US have a reason to live by the Golden Rule.
It's just that we don't. If we did the Crucifixion would never have needed to occur.
And who says everyone wants to live in peace?
And I do hope tortoise's Iterated Prisoner Dilemma claim is correct. It would give mathematical backing to a the teachings of Jesus.