In the Bible, the Book of Revelation (13:17-18) cryptically asserts 666 to be "the number of a man," associated with the beast, an antagonistic creature that appears briefly about two-thirds into the apocalyptic vision. The calculation of the number was a challenge that has inspired many would-be mystics and prophets ever since.
In modern popular culture, 666 has become one of the most widely recognized symbols for the Antichrist or, alternately, the Devil. Earnest references to 666 occur both among apocalypticist Christian groups and in explicitly anti-Christian subcultures, such as that surrounding some heavy metal bands. An appearance of the number 666 in contemporary Western art or literature is, more likely than not, an intentional reference to this number of the Beast symbolism. Such popular references to 666 are too numerous to list.
It is not uncommon to see the symbolic role of the integer 666 transferred to the digit sequence 6-6-6. Some people take the satanic associations of 666 so seriously that they actively avoid things related to 666 or the digits 6-6-6. This is known as hexakosioihexekontahexaphobia.