Revelation 13:16-18 (NIV):

“He also forced everyone, small and great, rich and poor, free and slave, to receive a mark on his right hand or on his forehead so that no one could buy or sell unless he had the mark, which is the name of the beast or the number of his name. This calls for wisdom. If anyone has insight, let him calculate the number of the beast, for it is man’s number. His number is 666.”

Revelation 14:9-11 (NIV):

“A third angel followed them and said in a loud voice: ‘If anyone worships the beast and his image and receives his mark on the forehead or on the hand, he, too, will drink of the wine of God’s fury, which has been poured full strength into the cup of his wrath. He will be tormented with burning sulfur in the presence of the holy angels and of the Lamb.  And the smoke of their torment rises for ever and ever. There is no rest day or night for those who worship the beast and his image, or for anyone who receives the mark of his name.’ “

Revelation 16:2 (NIV):

“The first angel went and poured out his bowl on the land, and ugly and painful sores broke out on the people who had the mark of the beast and worshiped his image.”

Revelation 19:20 (NIV):

“But the beast was captured, and with him the false prophet who had performed the miraculous signs on his behalf. With these signs he had deluded those who had received the mark of the beast and worshiped his image. The two of them were thrown alive into the fiery lake of burning sulfur.”