Revelation 13:16-18 NIV
It also forced all people, great and small, rich and poor, free and slave, to receive a mark on their right hands or on their foreheads, so that they could not buy or sell unless they had the mark, which is the name of the beast or the number of its name.
This calls for wisdom. Let the person who has insight calculate the number of the beast, for it is the number of a man. That number is 666.
NIV: New International Version