Revelation 13:16–18

16 And it makes everyone—small and great, rich and poor, free and slave—to receive a mark on his right hand or on his forehead,b 17 so that no one can buy or sell unless he has the mark: the beast’s name or the number of its name.

18 This calls for wisdom:D,c Let the one who has understanding calculateE the number of the beast, because it is the number of a person. Its number is 666.F