Whatever "it" is, it's primarily spiritual and associated with worshiping that which has as its animating force, something antithetical to the worship of the True God :
And the third angel followed them, saying with a loud voice, If any man worship the beast and his image, and receive his mark in his forehead, or in his hand,
The same shall drink of the wine of the wrath of God, which is poured out without mixture into the cup of his indignation; and he shall be tormented with fire and brimstone in the presence of the holy angels, and in the presence of the Lamb:
And the smoke of their torment ascendeth up for ever and ever: and they have no rest day nor night, who worship the beast and his image, and whosoever receiveth the mark of his name.
Revelation 14:9-11
Consider, by comparison, what happens BEFORE the mark of the beast is given :
And after these things I saw four angels standing on the four corners of the earth, holding the four winds of the earth, that the wind should not blow on the earth, nor on the sea, nor on any tree.
And I saw another angel ascending from the east, having the seal of the living God: and he cried with a loud voice to the four angels, to whom it was given to hurt the earth and the sea,
Saying, Hurt not the earth, neither the sea, nor the trees, till we have sealed the servants of our God in their foreheads.
Revelation 7:1-3
Unless one wants to assert that the servants of God in this passage receive an actual, literal mark in their foreheads - and that the seal of God is a literal implement used to create a (wax) seal, one is left with the clear understanding that what we might call the mark of God (in opposition to the mark of the beast) is an identification that is not physical. It is spiritual.
And there is Old Testament precedent for this. When God judged Israel in the days of Ezekiel and actually removed His presence from the temple, we find the assessment of the spiritual condition of Israel in Ezekiel 8. In that chapter, God gives Ezekiel a vision :
And it came to pass in the sixth year, in the sixth month, in the fifth day of the month, as I sat in mine house, and the elders of Judah sat before me, that the hand of the Lord God fell there upon me.
Then I beheld, and lo a likeness as the appearance of fire: from the appearance of his loins even downward, fire; and from his loins even upward, as the appearance of brightness, as the colour of amber.
And he put forth the form of an hand, and took me by a lock of mine head; and the spirit lifted me up between the earth and the heaven, and brought me in the visions of God to Jerusalem, to the door of the inner gate that looketh toward the north; where was the seat of the image of jealousy, which provoketh to jealousy.
And, behold, the glory of the God of Israel was there, according to the vision that I saw in the plain.
Ezekiel 8:1-4
The first thing I notice here is that God is showing Ezekiel something in the spirit that cannot be seen by the natural eye. In other words, even though He is taking Ezekiel in a vision to the temple, what he is being shown is not to be seen by walking into the temple physically. In short, it is a spiritual snapshot of Israel at the time. The second thing I notice is that there is that which is a place of residence called "the seat of the image of jealousy". I don't know precisely what that is, but from the statement in verse 4 ("...the glory of the God of Israel was there...") it seems to me that this is a place of the utmost spiritual significance. It is reflective of God's identification with the temple and it appears that the "image ... which provoketh to jealousy" is somewhat reflective of God's statement that not only is He a jealous God, but "Jealous" is one of His names (Exodus 34:14). To assault this seat of jealousy is (again, as I read it) to directly affront the all Holy One of Israel and should be something which should make one tremble. It is to strike at the very heart of God's presence. I would think it is akin (though maybe not directly analogous) to the unforgivable sin of blaspheming the Holy Spirit.
Having established this "seat of Jealousy", I note what Paul says about the man of sin to the Thessalonian church :
Let no man deceive you by any means: for that day shall not come, except there come a falling away first, and that man of sin be revealed, the son of perdition;
Who opposeth and exalteth himself above all that is called God, or that is worshipped; so that he as God sitteth in the temple of God, shewing himself that he is God.
2 Thessalonians 2:3-4
It seems to me to be more than a passing note that there is a recognition of a temple and where God dwells. And since - under the New Covenant - there is no more temple, it should (and this requires more dealing with the text, I agree) be clear that we have in view here a spiritual "coup" where the True God has been replaced by one who wants to pass himself off as God. So far from being a man walking into the temple in Jerusalem and saying "I am God...worship me" it seems to me to establish a spiritual impostor that is far more insidious - far more deceptive (deceiving even the elect - were it possible) and another line of support (though indirectly, I admit) for the mark of the beast being spiritual more than anything else.
The rest of the chapter (Ezekiel 8) is God taking Ezekiel progressively deeper in to the temple seeing greater and greater "abominations". It is God peeling back the layers of the depths of spiritual adultery to which Israel has degraded themselves. This is a vision (much like John's revelation was a vision) of the spiritual condition of a people and God's impending judgment upon them (much like John was seeing a revelation of Jesus Christ and how God was going to judge the world and what He would do with His own). These are physical pictures that illustrated greater natural realities of pervasive and deeply rooted idolatry. And, as we see, the way God judged Israel was similar in some respects to the way judgment is approached in Revelation.
So in going to the next chapter, there is one main point to be made here : that just like was expressed in Revelation 7, God marked His own people prior to judgment so that they would be spared.
And the glory of the God of Israel was gone up from the cherub, whereupon he was, to the threshold of the house. And he called to the man clothed with linen, which had the writer's inkhorn by his side;
And the Lord said unto him, Go through the midst of the city, through the midst of Jerusalem, and set a mark upon the foreheads of the men that sigh and that cry for all the abominations that be done in the midst thereof.
And to the others he said in mine hearing, Go ye after him through the city, and smite: let not your eye spare, neither have ye pity:
Slay utterly old and young, both maids, and little children, and women: but come not near any man upon whom is the mark; and begin at my sanctuary. Then they began at the ancient men which were before the house.
Ezekiel 9:3-6
So the first thing here (as in Revelation) was that those of Israel that were faithful to God were marked so as to not be hurt by the coming slaughter. The second - and most prominent - thing is that those who were spared were marked on the forehead by an angel. The third thing is that the judgment of God was held back until all those that were to be marked were actually marked. The fourth thing to note (though this doesn't necessarily directly support the spiritual mark idea) is that judgment started at "...my sanctuary.". Sounds an awful lot like Peter's statement :
For the time is come that judgment must begin at the house of God: and if it first begin at us, what shall the end be of them that obey not the gospel of God?
I Peter 1:17
Is Peter talking about a temple or about a people when he says "the house of God"? It should be clear by his use of "us" that it is the church - the body of believers - that is in view. Not a physical temple.
So when Revelation speaks about a mark on the forehead - I think it is clear that this is first of all a spiritual mark. If there is a physical mark someone takes, that is - I believe - only incidental. So anyone resisting (for example) letting themselves be injected with an RFID tracker may be missing the point. Not that I would say that they should allow such a thing, but something far more spiritual is in view here. Otherwise, anyone who doesn't (for whatever reason) get a chip in them is saved and anyone who does (for whatever reason - even if forced on them) is irredeemably lost. Just because of something implanted under their skin? I think (though I haven't heard his full answer) that's why MacArthur can't agree that taking the mark of the beast is totally unforgivable - because he recognizes the problem with being overly physical with this imagery.
