The Mark of the Beast is 666 Revelation 13

The Book of Revelation is the final book of the New Testament (and therefore the final book of the Christian Bible). Its title is derived from the first word of the Koine Greek text: apokalypsis, meaning "unveiling" or "revelation". The Book of Revelation is the only apocalyptic book in the New Testament canon.[c] It occupies a central place in Christian eschatology.