Revelation 13:12-18

12He exercises all the authority of the first beast before him, and he makes the earth and all those who dwell in it worship the first beast, whose fatal wound was healed.

13He performs great signs, even making fire come down from heaven in the sight of men.

14And he deceives those who dwell on the earth through the signs he is permitted to perform, telling those who dwell on the earth to make an image in honor of the beast who has the sword wound yet lived.

15The second beast was permitted to give life to the image of the first beast, so that the image of the beast could even speak and cause all who would not worship the image of the beast to be killed.

16He also causes all—the small and the great, the rich and the poor, the free and the slave—to receive a mark on their right hand or upon their forehead.

17And so no one can buy or sell unless he has the mark—either the name of the beast or the number of his name.

18Here is wisdom: let the one who has understanding calculate the number of the beast, for it is a number of a man, and his number is 666.