What is the origin of this idea that "six hundred and sixty-six" is an inauspicious number? Well, as with so many answers to mysterious questions, "it's in the Book", and in the very last book of the Christian Bible, namely The Apocalypse, or The Revelation (please don't show your ignorance by "misnoming" it "Revelations").
In Chapter 13 (yes, and I am not making this up) in the last verse (number 18, which is 3 times 6, I have noticed for the first time in my life, but, don't get me wrong, of course I have known that 3 x 6 = 18 for most of my life), we are told that the number of the Beast is "six hundred and sixty-six", that is, 6-6-6 (Revelation 13:18). Incidentally, interestingly, but not significantly, the Greek word for six is hex (be a witch or bewitch).
My interpretation of the visions and predictions in that chapter will be presented for your consideration here. Basically, I regard it as an updating of the prophecies of the Book of Daniel (chapters 7 to 12) for first-century Christians.
John the Seer (1.1-2) sees a monster, a "beast" (therion) coming up out of the sea (the place where evil lurks); it has seven heads with ten horns and each horn has a diadem on it (13.1) The beast was a composite creature, with body parts taken from a leopard, a bear, and a lion, and these were the three animals Daniel saw in a vision (Dn 7:1-8); they represent ancient world empires, and their rulers. The lion was Babylon and Nebukadressar (Nebuchadnezzar); the bear was the Median kingdom under Astyages the Mede; the leopard was the Persian empire under Cyrus the Great and his successors; and there was a fourth monster, with iron teeth and ten horns, representing Alexander the Great and his successors in the Grecian or Hellenistic empire; and a small horn (Dn 7.19-22) sprang up and made war on the saints (Israel), clearly Antiokhos IV (Antiokhos Epiphanes), who saw himself as a divine manifestation (an 'epiphany') and actively persecuted Jews. In this regard he was a model for Nero Caesar, who was also a Hellenist with a belief in his own divinity, and who likewise persecuted "the saints".
(Note that my unraveling of the web of mystery spun by the author of the Book of Daniel is set forth in detail in Colless 1992.)
My proposition is that Nero Caesar was the beast whose number was 666.
I am not the first one to notice this, but here is how it works: if you write the name in Hebrew letters (without vowels, as is the custom in Semitic languages) it is NRWN QSR; the letters were also used as numerals and if we give this set their numerical value it does add up to 666. That is according to the ancient occult art of "numerology".
Here is the "algebraic" equation:
N (50) + R (200) +W (6) + N (50) + Q (100) + S (60) + R (200) = 666 (DCLXVI)
I put some Roman numerals in there to lead on to the next point, to show that the Roman alphabet offers similar possibilities. The role of the apocalyptic "beast" is always persecution of the "saints", and this could apply to King Louis XIV, the Sun King, who slaughtered protestants.
Taking the name as LVDOVICVS we have:
L (50) + V (5) + D (500) + O (0) + V (5) + I (1) + C (100) + V (5) S (summa, total) = 666
(But wait, there's more to come)
Brian E. Colless, Cyrus the Persian as Darius the Mede in the Book of Daniel, Journal for the Study of the Old Testament 56 (1992) 113-126.