Mark of the Beast

The Number of the Beast (Greek: Ἀριθμὸς τοῦ θηρίου, Arithmos tou Thēriou) is a term in the Book of Revelation, of the New Testament, that is associated with the Beast of Revelation in chapter 13. In most manuscripts of the New Testament and in English translations of the Bible, the number of the beast is 666. In critical editions of the Greek text, such as the Novum Testamentum Graece, it is noted that 616 is a variant. In the oldest preserved manuscript as of 2017^[update], as well as other ancient sources like Codex Ephraemi Rescriptus, the Number of the Beast is 616 (χιϛ), not 666.

The Number of the beast is described in the passage of Revelation 13:15–18. The actual number is only mentioned once, in verse 18. In the Greek manuscripts, the Book of Revelation is titled The Apocalypse of John where this number is rendered in Greek numerical form as χξϛ, or sometimes literally as ἑξακόσιοι ἑξήκοντα ἕξ, hexakósioi hexēkonta héx, "six hundred and sixty-six". There are several interpretations-translations for the meaning of the phrase "Here is Wisdom, Let him that hath understanding count the number of the beast" where the peculiar Greek word ψηφισάτω (psephisato) is used. Possible translations include not only "to count", "to reckon" but also "to vote" or "to decide".

In the Textus Receptus, derived from Byzantine text-type manuscripts, the number 666 is represented by the final 3 letters χξϛ.

...
Wikipedia