The real reason 666 is the Devil's number

Different cultures have different no-no numbers, not to be confused with the abbreviation for "number," which is just "no." These no-no numbers get treated like no. 2 due to negative associations in their respective cultures. For instance, How Stuff Works observes that in hospitals and apartments in Japan often skip the 4th floor (jumping from 3rd to 5th) because the number four is the same as the word for "death" while the number 49 brings bad luck because it sounds like the Japanese words for "pain until death." Meanwhile, the worst number in the West among Christians is 666, Lucifer's favorite evil digits.

The Devil's number 666 has a fiery history

Just like the number four in Japan (and other East Asian countries for that matter), the triple sixes have a dark explanation. According to the University of Chicago, the the association between Satan and the triple-6's traces back to the days of Emperor Nero and the night Rome caught fire in 64 AD. Nero didn't didn't fiddle while Rome burned because fiddles didn't exist in ancient Rome, according to History. But many Romans believed Nero did a different kind of fiddling that intentionally set the city aflame. The blaze lasted six days, before "suspiciously flaring up again" and nearly two-thirds of Rome was lost along with innumerable lives. 

Nero was the real devil to Christians

Whether or not Nero was an arsonist, he was the first emperor to persecute Christians, whom he blamed for the blaze. He's also thought to have ordered the execution of the Apostles Paul and Peter. In turn, Christians began to view the emperor as the Antichrist. After his death in 68 AD, many Christians insisted "he was not dead and somehow would return." Over time, fear of zombie Nero subsided, and he became the Beast from the Sea (often confused with the Antichrist) of the Book of Revelation we all know and loathe.

666 is code for the real devil

Nero isn't named outright but referenced in coded messages. John writes in Revelation: "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." In Hebrew, much like in Greek, letters correspond with numbers. While the message was written in Greek, Hebrew numerals were the basis for the encoding. So when the Greek spelling of Nero is converted to Hebrew letters, you get 666.