Is it is a fire in so much it will be used to deceive all the world in receiving a mark of authority that is NOT of God?
in principle the satanic kingdom intends and tries to do exactly that by drawing and using (up to at least 33% of the) heavenly bliss and grace through its occult and idolatrous system(s)
If the term fire is metaphorical in usage, that is a diabolical plot to decieve the masses of the world, then how would the irreversible mark be realised, since people can repent after sinning, yet the action of taking the (definite article) mark is an unforgivable sin?
one of the biblical words for power/force is "fire", however, the word used in the manuscript from which the original text was taken, which in this case is the ancient greek word "πῦρ"(pyr), may have meant not only "fire" but also "power/force" i.e. in those times, though that meaning of the word is already obsolete/out of use nowadays - the reason for that was at least the fact that most of the ancient people had not had developed enough scientific knowledge and terminology, and therefore used bywords such as "wind", "fire", "tree", "sheep", "woman", etc.
the mark can be lasting for religious worshipers only if they persist in worshiping/ministering the wicked one/idols instead only of the true Lord God, then repentance may be more possible for people that have sinned a little, and more difficult/impossible for people that have sinned a lot
as for the unforgivable sin, spiritual as well as too much physical unrighteousness/lawlessness/wickedness leads its kings/servants to eternal fall - btw, there is no sin that is forgivable for all the coming time's infinity i.e. in the longest term of eternity - for that reason the best we can do is to try not to cause evil/harm/suffering to any person/soul at all
Is the mark something literal and tangible to the temple, that is the body, that has an irreversable consequence?
spiritual unrighteousness/lawlessness/wickedness also uses spiritualized physical forms of satanic/"beastly" worship/ministry such as wearing of the so-called "bindi" and "tilak(a)", burning of special incense sticks, having/attending altars with idolatrous statuettes, wearing astrological bracelets, witchcraft amulets, etc.
irreversible is every evil that was already caused to/inflicted on one person/soul or another, because neither is it possible that time be rewound or turned/brought back and the evil that happened to them be prevented, nor is it possible for any soul to avoid the eternal circle of existence, positions of and events with the souls in future
Blessings