This behavior came from a fear of having anything to do with the number 666, which some Christians have connected to Satan and see as a symbol of evil. read more
The 666 is an invention of the Christian Church to represent the Devil however as per The Satanic Rituals by Anton Lacey the Devil's real number is 9. This can be explained better in the chapter titled The Unknown Known. read more
666 (six hundred [and] sixty-six) is the natural number following 665 and preceding 667.. Six hundred and sixty-six is called the "number of the Beast" in (most manuscripts of) chapter 13 of the Book of Revelation, of the New Testament, and also in popular culture. read more