"And that no man might buy or sell, save he that had the mark, or the name of the beast, or the number of his name. Here is wisdom. Let him that hath understanding count the number of the beast: for it is the number of a man; and his number is Six hundred threescore and six."
...or at least so says Revelations 13:17–18. But why 666? What does the Bible have against that particular number? Why not 971? Or 380, which if you turn it sideways, kind of looks like a laughing devil with horns?
Turns out, it's a Pagan thing.
Way back in the day, the Babylonians had a lot of gods. 36 of them, to be exact. To each of these gods they gave a number, 1-36, based on how powerful they were. Made it nice and simple that way, I guess.
As with many polythestic religions, however, one Babylonian god - the Sun God - was believed to be the BEST god, and actually the father of most of the rest of the gods. Therefore, he had the power of all other gods, plus himself, put together. So whereas God #23 had was 23-powerful, and God #6 was 6-powerful... The Sun God was 1+2+3+4+5+6+...+36 powerful... or 666.
Then along came Christianity, with its one god (or three gods, depending on how you look at it), and of course anything so ridiculous as 36 gods immediately became Pagan idolatry. Devil worship. And 666, the old Pagan Sun God, was the head devil of them all.
Interestingly, early people fearing the Babylonian Gods sometimes carried around amulets with all 36 Gods' numbers listed on them, arranged in a 6x6 matrix so that every row and column added up to the same total (for extra protection). Today, we call these arangements "Magic Squares".
Satan's Sokuku puzzle
Bonus factoid: for the same 1-36 sumation reasons, the numbers on a Roulette Wheel also add up to 666.
Proving once and for all that gambling - and math - is the Devil's tool.