Revelation 13:17-18

Amplified Bible, Classic Edition

¹⁷ So that no one will have power to buy or sell unless he bears the stamp (mark, inscription), [that is] the name of the beast or the number of his name.

¹⁸ Here is [room for] discernment [a call for the wisdom ^[a]of interpretation]. Let anyone who has intelligence (penetration and insight enough) calculate the number of the beast, for it is a human number [the number of a certain man]; his number is 666.