The Mark of the Beast - 666

Rev 13:16 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:
Rev 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.
Rev 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.

This mark will come into play during the midst of the Tribulation hour and is identified with the first beast or Antichrist. The false prophet will use this system to control those on the earth to worship the Beast. Those who do not take the mark will not be able to buy or sell. There has been much speculation throughout the centuries concerning the number 666 and which infamous personality the number may point to, although during the Tribulation his personage will be well known by all.

Note: Typologically, the three Hebrew children in Daniel 3 are a type of the Jewish remnant who will be sealed (See Rev. 7) and preserved through the Tribulation hour. There were six instruments involved in the worship of Nebuchadnezzar's image in the Plain of Dura, the image was six cubits wide and sixty cubits tall (666). They were to be forced on pain of death to bow down and worship the image or be thrown alive into the fiery furnace. There is more here, but the idea is that this points to Revelation 13 where the false prophet initiates a satanic religion during the last half of the Tribulation period. By this time the world church of Revelation 17 has been destroyed.