Revelation 13:16-18

The beast forced all the people, small and great, rich and poor, slave and free, to have a mark placed on their right hands or on their foreheads. No one could buy or sell without having this mark, that is, the beast's name or the number that stands for the name. This calls for wisdom. Whoever is intelligent can work out the meaning of the number of the beast, because the number stands for a human name. Its number is 666.