Revelation 13:16-18

He made everyone, whether weak or powerful, rich or poor, free or slave, receive a mark on their right hand or on their foreheads. Nobody was permitted to buy or sell except those who had the mark, which was the name of the beast or the number of his name. Wisdom is needed here. Whoever has understanding should calculate the number of the beast, for it is the number of a man. His number is 666.