Revelation 13:16-18

New Catholic Bible

¹⁶ It also forced all the people, both small and great, both rich and poor, both free and slave, to be branded on the right hand or on the forehead. ¹⁷ No one could buy or sell anything except one who has been branded with the name of the beast or with the number of its name.

¹⁸ There is wisdom here. Let anyone who has understanding calculate the number of the beast, for it is the number of a person. The number is six hundred and sixty-six.