Revelation 13:16-18

All people were forced to put a mark on their right hand or forehead. Whether they were powerful or weak, rich or poor, free people or slaves, they all had to have this mark, or else they could not buy or sell anything. This mark stood for the name of the beast and for the number of its name. You need wisdom to understand the number of the beast! But if you are smart enough, you can figure this out. Its number is 666, and it stands for a person.