666 is cryptically referred to in Rev 13:16-18 as the “mark,” “name,” or “number” of the Beast, said to mark the forehead or the right hand of all who buy or sell. Mainstream biblical scholars interpret the Beast as a symbol for the Roman Empire, an image that conveys governmental control and the extended, evil reach of the empire in commerce.
Given that Greek and Hebrew letters possess numerical equivalents, “the number of its name” (Rev 13:17) is thus code for a word. The most likely original candidate for the name is Nero Caesar, which yields 666 when translated from Greek into Hebrew. The identification of Nero also yields 616 in another common spelling, which is the number of the beast given in some critical Greek manuscripts of the New Testament.
After 1611, the English translation of the King James Version of the Bible identified the number as “six hundred  threescore [3 × 20] and six ,” yielding 666 as the number that has popularly been associated with the “mark of the Beast.” In English-influenced cultures, this number also came to be identified as the number of the Antichrist, a term that never appears in the Book of Revelation but rather derives from the epistles 1 and 2 John, where it appears as both “one who opposes Christ” (1John 2:18, 1John 2:22; 1John 4:3; 2John 1:7) and “those who oppose Christ” (1John 2:18). Here, the term mainly characterizes the opposing side of a dispute that originated as an intra-Christian debate on the nature of Christ. However, by the fourth century, the Antichrist functioned as an umbrella symbol combining the book of Revelation’s major symbols of evil: the Dragon (ultimate cosmic evil, Satan), the First Beast (the Roman Empire), and the Second Beast (the Roman priestly and governmental apparatus).
Due to its association with the Antichrist and the suppleness of the symbol, 666 came to be identified throughout its long history with whatever enemy a given apocalypticist may have had in mind, including Catholics, the papacy, Freemasons, and leaders of Israel and of Islam. In such cases, the number 666 loses its original biblical context entirely and serves simply to justify fears or hatred.
In English-influenced popular culture, the number 666 has taken on a wide field of associations with evil. Rock and punk music purposefully perpetuate it, sometimes with tongue in cheek, as a symbol of bad behavior or even Satanism (for example, Iron Maiden’s album 666). As a cultural countermovement to those who fear the identification of 666 with a contemporary figure or object, some participate in lighthearted mockery of the “evil” number through products such as 666 vodka and the 666 energy drink.
By contrast, certain groups have straightforwardly adopted the numerical symbol for their self-identification. In a standard, racist tattoo of the Aryan Brotherhood gang, the number is superimposed on a shamrock. The leader of Creciendo en Gracia Ministries, who claims to be both the “Man Jesus Christ” and the “Antichrist,” has adopted the number as a symbol for church; many members have tattooed it on themselves and on their children.
In recent decades, conspiracy-minded individuals who combine idiosyncratic interpretations of the Bible with fears that evil governmental or religious forces are overtaking society have variously interpreted 666 as a reference to the United Nations, some presidents of the United States, the Washington Monument, and the European Union. Given the commercial associations with 666 in Revelation, such interpreters also commonly identify the flexible symbol with the modern barcode system, the “www” of the Internet’s prefix for the World Wide Web, RFID seals used for tracking and identification, and smart cards used for cashless swiping. Such conspiratorial speculation unhelpfully feeds the phobias of some, whose fear of the number 666 has yielded a new term: “hexakosioihexekontahexaphobia.”