Revelation 13:16-18

It forces everyone—the small and great, the rich and poor, the free and slaves—to have a mark put on their right hand or on their forehead. It will not allow anyone to make a purchase or sell anything unless the person has the mark with the beast’s name or the number of its name. This calls for wisdom. Let the one who understands calculate the beast’s number, for it’s a human being’s number. Its number is six hundred sixty-six.