Revelation 13:16-18. The second beast (false prophet) "causes all, both small and great, both rich and poor, both free and slave, to be marked on the right hand or the forehead, so that no one can buy or sell unless he has the mark, that is, the name of the beast or the number of its name. This calls for wisdom: let the one who has understanding calculate the number of the beast, for it is the number of a man, and his number is 666." The mark is the sign of ultimate allegiance to the beast system — a counter-seal to the seal of God placed on the foreheads of the 144,000 (Revelation 7:3).
The Mark of the Beast has generated more speculative interpretation than any other symbol in Revelation. Several interpretive keys help. (1) 666 likely identifies Nero. Using Hebrew gematria on "Nero Caesar" (Neron Qesar, נרון קסר) gives 666. This fits the 1st-century context of Revelation's composition (likely AD 95, with Nero as the archetype of persecuting emperor). The variant reading 616 — found in some ancient manuscripts — works if one uses the Latin "Nero Caesar" instead of the Greek spelling, confirming that the number referred to Nero. (2) Recurring type. Every generation has its Nero-types — regimes demanding ultimate allegiance that conflicts with Christ. The mark of the beast is the symbolic expression of the choice every believer eventually faces: Christ or Caesar. In some ages this choice is literal (Roman, Soviet, Chinese persecution); in others it is cultural (refusing the moral demands of a post-Christian culture that excludes faithful Christians from employment, platform, and public life). (3) Right hand and forehead. Deuteronomy 6:8 commanded the people of God to bind His Word "as a sign on your hand, and they shall be as frontlets between your eyes" — the beast's mark is the satanic counter-ritual, marking allegiance on the same body-parts. (4) "Buy or sell". Economic exclusion for dissenters has been a favored tool of totalitarian regimes throughout history and is increasingly an option in digitally-integrated economies. The passage is a warning to every generation; speculation about specific technologies (microchips, etc.) has been repeatedly embarrassed by history. Focus on the principle, not the prediction: Christ or Caesar.