I don't remember if there is, actually, at least not explicitly. But even if there was not an explicit verse saying that He protects believers (or the elect, depending on your view of free will versus God's sovereignty) from ever going back to unsaved status in any way, just to me common sense rules out that people who are already believers can just take some sort of physical mark by accident - thinking it a mere identification device - and "boom!" suddenly they worship man/Satan and no longer belong to Christ.
Matthew 24:22-24 I think might be the verses I am thinking of?
Matt 24:22-Matt 24:24 NASB
Unless those days had been cut short, no life would have been saved; but for the sake of the elect those days will be cut short. Then if anyone says to you, ‘Behold, here is the Christ,’ or ‘There He is,’ do not believe him. For false Christs and false prophets will arise and will show great signs and wonders, so as to mislead, if possible, even the elect.
as for sovereignty/ free will
it is a paradox, a duality. Human Responsibility and Divine Sovereignty somehow are both responsible. I am not a master of the theology, but the more I study it, the more it matches what I read in scripture, even if I do not understand the paradox.
Same with the trinity.
Same with the duality of light. Even if I do not understand it, the more I study it, the more it matches the experiments.
as for the mark... I highly doubt it will tangible. As much is obvious that the mark of the beast, the number of his name, 666 is not literal. it is symbolic. Just like with the dimensions of the statue of Nebuchadnezzar
7 represents perfection, godliness, holiness, etc.
6 represents humanity, imperfection, falliness.
in the numeric system of the day, 6 is to 7 what 9 is to 10 in our system. Their number system was based on sixes. saying that his mark/number is 666 is like saying 999. it is as close as you can't get to 7 (or 10) without actually getting there. It is a picture of a being who is striving to reach the place of God, but never making it.