Revelation 13:18 says, “Here is wisdom. Let him who has understanding calculate the number of the beast, for the number is that of a man; and his number is 666.”
The numbers 666 have been associated to the mark of the beast in the bible. It is supposed to be the devils number. 666 also could mean 6 protons, 6 electrons and 6 neutrons, but it’s mainly associated with the number of the beast which is six hundred and sixty-six.