Mark

Romans 4:11
[4:11] And he received the sign of circumcision, a seal of the righteousness of the faith which he had yet being uncircumcised: that he might be the father of all them that believe, though they be not circumcised; that righteousness might be imputed unto them also:

Revelation 13:17
[13:17] And that no man might buy or sell, save he that had the mark, or the name of the beast, or the number of his name.

Revelation 14:9-11
[14:9] And the third angel followed them, saying with a loud voice, If any man worship the beast and his image, and receive his mark in his forehead, or in his hand, [10] The same shall drink of the wine of the wrath of God, which is poured out without mixture into the cup of his indignation; and he shall be tormented with fire and brimstone in the presence of the holy angels, and in the presence of the Lamb: [11] And the smoke of their torment ascendeth up for ever and ever: and they have no rest day nor night, who worship the beast and his image, and whosoever receiveth the mark of his name.

Revelation 7:2-3
[7:2] And I saw another angel ascending from the east, having the seal of the living God: and he cried with a loud voice to the four angels, to whom it was given to hurt the earth and the sea, [3] Saying, Hurt not the earth, neither the sea, nor the trees, till we have sealed the servants of our God in their foreheads.