Revelation 13:11-18

Then I saw another beast which rose out of the earth; it had two horns like a lamb and it spoke like a dragon. It exercises all the authority of the first beast in its presence, and makes the earth and its inhabitants worship the first beast, whose mortal wound was healed. It works great signs, even making fire come down from heaven to earth in the sight of men; and by the signs which it is allowed to work in the presence of the beast, it deceives those who dwell on earth, bidding them make an image for the beast which was wounded by the sword and yet lived; and it was allowed to give breath to the image of the beast so that the image of the beast should even speak, and to cause those who would not worship the image of the beast to be slain. 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, 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. This calls for wisdom: let him who has understanding reckon the number of the beast, for it is a human number, its number is six hundred and sixty-six.