Let me first ask you a simple question. Do not the scriptures EXPLICITLY warn you that should you take the mark of the beast, you will suffer eternal torment? Would you still have Jesus' 'imputed' righteousness then?
What makes you think sealed believers would accept the mark of the beast?
If you evade answering this question, possess a crystal ball and are 100% certain that you would never take the mark and thus deny Christ, just remember that Peter who walked and talked with Jesus daily was absolutely certain that he would not deny Christ; yet we all know what happened to Peter. And why would you not take it? After all, don't you believe that you can never lose you justification??
Don't need a crystal ball as the remaining chapters of Revelation show those who take the mark are not God's elect. In fact, we have this:
Revelation 7: ESV
1After this I saw four angels standing at the four corners of the earth, holding back the four winds of the earth, that no wind might blow on earth or sea or against any tree. 2Then I saw another angel ascending from the rising of the sun, with the seal of the living God, and he called with a loud voice to the four angels who had been given power to harm earth and sea, 3saying, “Do not harm the earth or the sea or the trees, until we have sealed the servants of our God on their foreheads.”
So it seems God has that one under control too.
That is why we are admonished to continue in the faith - if indeed you continue in the faith, stable and steadfast, not shifting from the hope of the gospel that you heard, which has been proclaimed in all creation under heaven, and of which I, Paul, became a minister. Col 1:23
The word "if" indicates only the possibility - not the certainty - that one will in fact continue in the faith. "If indicates a condition - stable, steadfast, not shifting - that must be met by us in order to continue in the faith.
The 'if' is related to the Gospel here right? Meaning the Gospel of Grace. And what is this hope he speaks of? Yes eternal life. What did Jesus say about His sheep and eternal life?
Therefore, Paul is saying "if" someone does not stay steadfast in the Gospel which is hour hope. If one does not believe the Gospel and the sure hope of eternal life, what do we call such people....Right unbelievers. It means they don't believe in the Gospel to save.
If your point is Paul is addressing 100% born again believers in these churches, I think that is a huge assumption. Jesus told us there would be tares among the wheat. It's why Paul uses "if" and in one case "test yourself to see if you are truly in the faith" and also "But if Christ is in you" type statements. He makes contrasts between what is genuine salvific faith and what fruits or lack thereof are not.
The Apostle Paul warned the brethren in Rome that if they live according to the flesh (habitual sin), they will die (Rom 8:13). The Apostle James wrote: "My brothers, if anyone among you wanders from the truth and someone brings him back, let him know that whoever brings back a sinner from his wandering will save his soul from death and will cover a multitude of sins. " Js 5:19-20
The Apostle John wrote "Little children, let no one deceive you. Whoever practices righteousness is righteous, as he is righteous. Whoever makes a practice of sinning is of the devil, for the devil.... 1 Jn 3:7-8
Once again, there is no indication one who is born again, regenerated in the Spirit falls into these categories. What did Jesus say about His sheep again?
The numerous warnings in Scripture against apostatizing from the faith are written to believers; not unbelievers.
It can be both. For we know from Christ many are called but few are chosen and also the parable of the wheat and tares not to mention the parable of the sower/soils. Add to that, what did Jesus say to those who claimed Him but were not His? He said "I never knew you."
It is impossible for an unbeliever to depart from the faith since he/she was never a part of the faith to begin with.
In the same chapter where Jesus says "I never knew you" He also discussed those who were influenced by false teachers and were deluded.
Remember to take these "proof texts" in isolation of other teachings is just pitting Scriptures against each other and thus you are creating a contradiction where there is none.
Jesus said His sheep will never perish and they would be given eternal life. From that standpoint we must regard all warnings.
The notion that true believers can never die spiritually and lose their justification/salvation is a lie.
Actually Jesus said His sheep will never perish or be plucked from the Father or Him.
It is just another lie from Satan - the same lie that the serpent spoke to Eve in Gen 3:4 "You will not certainly die," the serpent said to the woman."
Don't believe the lie.
Jesus did not lie.