Revelation 13:16-18 16 Also it causes all, both small and great, both rich and poor, both free and slave, to be marked on the right hand or the forehead,  17 so that no one can buy or sell unless he has the mark, that is, the name of the beast or the number of its name.    18 This calls for wisdom: let the one who has understanding calculate the number of the beast, for it is the number of a man, and his number is 666.