I thought I would repost the intermittent Quora questions that I answer here.
Where did the “devil sign” 666 come from? For that, we have to learn a bit about the history of written language.
I II III IV V VI VII VIII IX X
You probably recognize the above as Roman Numerals.
1 2 3 4 5 6 7 8 9 10
You recognize the above, but do you know what they’re called? Arabic Numerals. Yes, our number lettering system comes from Arabs. This is why Romans used Roman Numerals: They just used the letters of their own alphabet to double as numbers.
Greeks civilization peaked before Roman civilization, so the Greeks did not use Roman Numerals. They used their own letters for numbers:
Α B Γ Δ E ΣΤ Z H Θ I
And Jews used their own letters for numbers as well (note– Hebrew is read from right to left):
א ב ג ד ה ו ז ח ט י
So this means that spelling someone’s name out has a numeric equivalent. If, for example that makes sense, you know someone named “Vix”, this is 5 + 9 (V + IX) or 5 + 1 + 10 (V + I + X). You could talk in code about “fourteen” or about “sixteen” and people could infer that you’re talking about Vix.
So the “devil sign” isn’t actually 6–6–6, or three sixes. It is really six hundred and sixty six. Moreover, that isn’t the only “devil sign”; there are some manuscripts of Revelation that have six hundred and sixteen instead of six hundred sixty six. Just like in the person named “Vix” example above, adding up someone’s name might result in different totals depending on how you parse the letters of their name.
With that being said, the name of god in Hebrew is YHWH or יְהוָה. Y is the number 10, H is the number 5, and W is the number 6. This adds up to 10 + 5 + 6 + 5, or 26.
 Arabic numerals – Wikipedia