What is the significance of '666'?
“And he causeth all, both small and great, rich and poor, free and bond, to receive a mark in their right hand, or in their foreheads: 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.” (Revelation 13:16-18) Thus the significance of 666 is that it is the number of the beast, which is Rome. It is the number of the name of a man (probably in Hebrew), who was probably known to the original readers of the book of Revelation. Possibly it is the numeric equivalent of the Hebrew name of one of the Roman emperors. This would fit with it being also the number of the beast, since the Emperor and the Roman government were considered synonymous.
See also my article DCXLVI for more about 666.