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Definition: The mark of the beast is the sign of the Antichrist, and is mentioned in Revelation 13:15-18:
The second beast was given power to give breath to the image of the first beast, so that the image could speak and cause all who refused to worship the image to be killed. It also forced all people, great and small, rich and poor, free and slave, to receive a mark on their right hands or on their foreheads, so that they could not buy or sell unless they had the mark, which is the name of the beast or the number of its name.
This calls for wisdom. Let the person who has insight calculate the number of the beast, for it is the number of a man. That number is 666. (NIV)
It seems there are as many interpretations of this passage as there are Christian denominations. Some believe these verses refer to a tattoo, a brand, or even a microchip implant. Theories also abound about the number 666.
When the Apostle John wrote Revelation, about 95 A.D., numerical values were sometimes assigned to letters as a sort of code. A common theory about 666 is that it was the numerical total for the name Nero Caesar, a Roman emperor who persecuted Christians. Tradition says Nero had the Apostle Paul beheaded about 64 or 65 A.D.
Numbers are often used symbolically in the Bible, the number 7 representing perfection. The Antichrist, a man, has the number 666, which continually falls short of perfection. The letters in Jesus Christ supposedly total 888, which goes beyond perfection.
Recently, many claim that the implant of medical or financial electronic ID chips are the mark of the beast. Others point to credit or debit cards. While those items may be an indication of what is to come, Bible scholars agree that the mark of the beast will be a recognizable sign of those who have voluntarily chosen to follow the Antichrist.
Bible References to the Mark of the Beast: Revelation 13:15-18; 14:9, 11; 15:2; 16:2; 19:20; and 20:4.
Also Known As: 666, 666 the number of the beast, 666 satan, 666 beast, beast 666.
The mark of the beast on the forehead or right hand may be literal or may symbolize allegiance of thought and action to the Antichrist.
(Sources: New Bible Commentary, edited by G.J. Wenham, J.A. Motyer, D.A. Carson, and R.T. France; The Abingdon Bible Commentary, edited by F.C. Eiselen, Edwin Lewis and D.G. Downey; ESV Study Bible; and gotquestions.org.)