The Roman emperor Constantine, before a decisive battle, had a vision of a burning cross in the sky and heard or saw these words. He had the sign painted on his soldier's shields and when he won the battle, promptly converted to Christianity and made it the official religion of the empire. Or so the story goes.

Close! The symbol Constantine dreamed of was the agglomeration of the greek letters Chi and Rho, a symbol that remains important in some denominations of Christianity to this day. Chi and Rho are the first two letters of the word "Christ", as the Greeks spelled it.