Jamieson, Fausset and Brown on Revelation 13:17

17. And--So A, B, and Vulgate read. C, Irenæus, 316, Coptic, and Syriac omit it.

might buy--Greek, "may be able to buy."

the mark, or the name--Greek, "the mark (namely), the name of the beast." The mark may be, as in the case of the sealing of the saints in the forehead, not a visible mark, but symbolical of allegiance. So the sign of the cross in Popery. The Pope's interdict has often shut out the excommunicate from social and commercial intercourse. Under the final Antichrist this shall come to pass in its most violent form.

number of his name--implying that the name has some numerical meaning.