What is the mark of the beast?
The mark of the beast is a mark placed in the right hand or forehead by the Antichrist, and without it no one will be allowed to buy or sell; the number of the beast that goes with this mark is 666.
Scripture teaches that during the reign of the Antichrist, he will cause all people, small and great, rich and poor, free and bond, to receive a mark in their right hand or in their foreheads. Revelation 13:16-17 says, "And he causeth all, both small and great, rich and poor, free and bond, to receive a mark in their right hand, or in their foreheads: 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." This mark is tied directly to his name or to a number, and that number is given in Revelation 13:18: "Here is wisdom. Let him that hath understanding count the number of the beast: for it is the number of a man; and his number is Six hundred threescore and six."
The purpose of this mark is economic control. The Antichrist causes craft to prosper in his hand, meaning he restores prosperity to those who take his mark, but withholds his support from anyone who refuses it. Those without the mark will not be able to buy or sell, which creates the real threat of starvation for anyone who will not submit to worshipping the beast.
Taking this mark carries eternal consequence. Revelation 14:9-11 warns that whoever worships the beast and his image and receives his mark in the forehead or hand will drink of the wine of the wrath of God, poured out without mixture, and will be tormented with fire and brimstone in the presence of the holy angels and of the Lamb, with the smoke of their torment ascending forever, having no rest day or night. This stands in direct contrast to those whose names are written in the Lamb's Book of Life, who refuse the mark and remain faithful, holding to the patience and the faith of the saints.
The scriptures
And he causeth all, both small and great, rich and poor, free and bond, to receive a mark in their right hand, or in their foreheads: 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. Here is wisdom. Let him that hath understanding count the number of the beast: for it is the number of a man; and his number is Six hundred threescore and six.
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, 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: 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.
He that leadeth into captivity shall go into captivity: he that killeth with the sword must be killed with the sword. Here is the patience and the faith of the saints.
From the books
This answer is drawn from these passages in the Mary Banks Faith Library.