It is based on a Bible passage Revelation 13-18:
This calls for wisdom. If anyone has insight, let him calculate the number of the beast, for it is man's number. His number is 666.
This is one of the most famous Bible quotes, made so both by The Omen film and by Iron Maiden's song The Number Of The Beast, which both quote it in different forms.
Revelation 12 v 12 has this passage:
Therefore rejoice, you heavens and you who dwell in them!
But woe to the earth and the sea, because the devil has gone down to you!
He is filled with fury because he knows the time is short.
In the Iron Maiden song, they splice these two passages together, but from a different translation:
"Woe to you, Oh Earth and Sea, for the Devil sends the
beast with wrath, because he knows the time is short...
Let him who hath understanding reckon the number of the
beast for it is a human number, its number is Six hundred and
sixty six."
