Mark of the Beast

It may refer to one's will. "...a mark that identifies someone as being faithful to the Antichrist's kingdom." Full text: What is the mark of the beast (666)?

See:

Pronouncement of Judgment on those who receive the mark of the beast by J. Vernon McGee

Whatcha gonna talk about next? Demon locusts? "You know why the Tribulation is going to be a terrible time? Because all the bound demons in hell are going to be turned loose to add to the ones that are already running all over the earth. And it says they were commanded in verse 4 that they should not hurt the grass of the earth, neither any green thing, neither any tree but only those men who have not the seal of God on their foreheads. The demons aren't going to do anything to the creation, they're just going to wipe out men. And they don't even give the privilege to kill them in the fifth trumpet. They can't kill them it says in verse 5, they can only torment them five months and their torment was like the torment of a scorpion when he strikes a man. And in those days men shall seek death and not find it and shall desire to die and death will flee from them. They're going to get stung like the sting of a scorpion, the locust plague of demons are going to cross the globe torturing men for five months, unable...men will be unable to find relief even in death. And then you have a description in verses 7 to 10 of these demonic beings, symbolic language. And it says in verse 11 they had a king over them and the king is the angel of the bottomless pit whose name in the Hebrew tongue is Abaddon, means destroyer, and in the Greek tongue, Apollyon means destroyer..." Full text: A Jet Tour Through Revelation by John MacArthur

Mark of the Beast