The mark of the beast is referenced, in the KJV and other Bible translations, only six times in the New Testament. All these references to a mark appear in the book of Revelation (Revelation 13:17, 14:11, 15:2, 16:2, 19:20 and 20:4). Whatever the mark is, it is something people receive or possess on their "foreheads" or "right hands" (Revelation 13:16). In the highly symbolic language of this book, these phrases represent a person's choices (“forehead”) as well as the actions they lead them to take.
The phrase "mark of the beast" is recorded only twice in Scripture. Revelation 16:2 states, ". . . and an evil and grievous sore fell upon the men who had the mark of the beast." Revelation 19:20 states, ". . . and with him the false prophet . . . by which he had deceived those who received the mark of the beast." The mark of the beast will not so much be something people receive, as it will be a demonic confirmation of their deceptions. In the end time, those who truly obey God will reject the one-world system of religious lies that will be enforced and pay with their lives. All others, who spiritually have the mark of the beast in their hearts (they are unconverted), will manifest this attitude by doing what they are told. Their actions will be a visible testament of their obedience.
The world has been inundated with the idea that the mark of the beast is linked to some visible manifestation of the number 666. Movies (The Omen, etc.), games, television programs, books, Web sites, talk show hosts and so on offer countless ideas regarding how this number will be used to accomplish Satan's goals. Will people be forced by the beast to receive a computer chip in their body with 666 embedded in it? Will everyone have to get a tattoo or other mark on their arm with 666? Will this number soon appear on the licenses of drivers or on our money? Will it be used to create a new cashless society? The theories are endless.
Here is something to consider. Satan did not deceive one third of the angels (Revelation 12:4) by first telling them that everything he would say would be a LIE. He did not warn them, in advance, that the ideas he espoused were meant to benefit HIM at the cost of their eternal lives. He is evil but not stupid. He operates based on subtlety and twists of the truth, pretending to be righteous in order for his deceptions to be the most effective (2Corinthians 11:13 - 15). He is not dumb enough to warn humans regarding when he might appear, how to identify him as the Beast, or how he will get mankind to worship him. He will NOT use "666" as his calling card of destruction!
One clue regarding what is the true mark of the beast comes from the fact that those who OBEY God, meaning those who keep HIS commandments (not their version of them), are considered righteous and acceptable to Him (Revelation 14:11 - 12). In the Old Testament, the SIGN between God and His people is the keeping of the seventh day Sabbath (Exodus 32:13). Obedience to it requires a willingness to believe it is the truth (the forehead) and to act on that belief (resting and worshipping the Eternal from Friday to Saturday sunset). It is also the chief commandment that those who pretend to be Christians can either easily dismiss or change to suit themselves (e.g. Sunday worship).
The true mark of the beast is a heart that breaks God's laws and the actions that follow those choices. Man's unwillingness to listen to and OBEY their Creator will be reinforced and rewarded in the period before Jesus' Second Coming. It will be a time when the devil will appear, as the world's pseudo-savior, to create his perverted version of the Millennium.