Revelation 13:
13:16 And he causeth all, both small and great, rich and
poor, free and bond, to receive a mark in their right hand, or in their foreheads:
13:17 And that no man might buy or sell, save he that had the mark, or the name of the beast, or the number of his name.
13:18 Here is wisdom. Let him that hath understanding count the number of the beast: for it is the number of a man; and his number is Six hundred threescore and six.
There was a committee that determined what books were in the Bible. Revelation was a problem because it seemed important it had been submitted. But.......no one could agree on what it said or meant. It got grandfathered in