Rev 13:16-18 describes a terrible time to come when the beast will force all people (great and small, rich and poor, free or 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 associated with the name or number of the beast (666).
Much ink has been spilled debating what this mark will look like or what the number 666 represents. Some have considered the number to represent one of the Roman Emperors like Nero or Caligula or Titus. Thus, antichrist would be Nero brought back to life. Others have claimed the Antichrist as some current historical figure of their day like Hitler, Mussolini or even Napoleon. Some have thought it stood for the word “Latin” telling us he would be a Roman. All of these simply show the endless speculations which have occupied the minds of students but have not really answered this impossible question.
It is probably wisest to understand the number in symbolic terms, seven being the number of perfection and six, therefore, being a number associated with imperfection. Though the beast claims to be God, he is not. He is only a man, an evil and demonically controlled man, or a state system representing evil (Rev 13:4, 2 Thess 2:4). The three sixes may allude to a Satanic trinity (Satan or the dragon, seeking to replace the Father, the beast seeking to replace the Lord Jesus Christ, and the false prophet, seeking to replace the Holy Spirit). What is clear is that all that comes from Satan can never reach perfection (symbolised by the number seven.) Man promises peace but brings war; life, but brings death; liberty, but brings slavery. There is nothing the world offers which can match the goodness and grace of God, and we need much wisdom to avoid deception in all ages.