I personally believe that those who have confessed Jesus Christ as their Savior in this world, but fall away, especially at the end, will... still be saved, eventually. Notice I said eventually. I don't believe all the NT Messages to those who believe on The Father through His Son's Blood shed upon the cross were given in vain.
But can we omit the NT warnings our Lord Jesus and His Apostles gave about staying in Him and His Word and not to be deceived? I don't think so. Likewise, can we just throw away the examples He gave of those servants He will shut the door upon? I don't think we can discard those Scriptures either, although that's exactly what many try to do.
Can we also throw away what Apostle Paul said about those who keep doing the sins of the flesh he listed in Galatians 5, whom he also said shall not inherit the Kingdom of God? I don't think so. All those Scriptures must be weighed too.
Remember about the believer among the Church at Corinth that was having sexual intercourse with his own mother which Paul cast out and rebuked the members there for not doing so:
1 Cor 5:1-5
5:1 It is reported commonly that there is fornication among you, and such fornication as is not so much as named among the Gentiles, that one should have his father's wife.
2 And ye are puffed up, and have not rather mourned, that he that hath done this deed might be taken away from among you.
3 For I verily, as absent in body, but present in spirit, have judged already, as though I were present, concerning him that hath so done this deed,
4 In the name of our Lord Jesus Christ, when ye are gathered together, and my spirit, with the power of our Lord Jesus Christ,
5 To deliver such an one unto Satan for the destruction of the flesh, that the spirit may be saved in the day of the Lord Jesus.
KJV
Paul told them to cast that believer out from among them, which no doubt was about isolating that sinner from the Church; that so his spirit may be saved in the day of the Lord Jesus??? What about that idea in bold red? If we look closely in God's Word, we notice that sinners are still present during Christ's future Millennial reign, but in a place of separation our Lord Jesus called the "outer darkness".
In Matt.25, we are shown the unprofitable servant will be cast to the "outer darkness".
Where is that "outer darkness" in Christ's future Kingdom?
Rev 22:14-15
14 Blessed are they that do His commandments, that they may have right to the tree of life, and may enter in through the gates into the city.
15 For without are dogs, and sorcerers, and whoremongers, and murderers, and idolaters, and whosoever loveth and maketh a lie.
KJV
That is the "outer darkness" our Lord Jesus warned about. In all three NT examples of the "outer darkness", Jesus revealed also there in that place will be weeping and gnashing of teeth (Matt.8:12; 22:13; 25:30).
To me, that "outer darkness" represents the hell of that future Millennial time, i.e., the abode of the wicked on earth, outside the gates of the beloved city.
The one of 1 Cor.5 that was having sex with his mother, I believe that is where Paul was talking about that sinner would be during "the day of the Lord Jesus", which is about the day of Christ's coming and start of His "thousand years" reign of Rev.20. It's possible that sinner was forgiven and reconciled to the Church at Corinth, but if not, I think the "outer darkness" is where he would be in Christ's future Millennial reign.
Thus, what I also see with this, is pointing to the ability of one's spirit like that sinner still possibly being saved by Jesus in that time (it's up to Him though I must affirm).
So my bottom line question on this might be, where do we as believers want to be during that time of our Lord Jesus' future thousand years reign with His elect? Inside the gates of the beloved city and have right to the tree of life, or outside those gates in the "outer darkness"? For the deceived and fallen away, I believe that "outer darkness" is where they will wind up if they do not turn back to Jesus and become more focused in The Faith (I do not exclude myself in that possibility either IF I fall away from Jesus Christ).
This would explain why our Lord Jesus in Rev.16:15 within the 6th Vial timing, warned His Church that He comes "as a thief", and blessed are those who watch, and keep their garments, lest they walk naked, and they see his shame. Those garments represent the garments of Salvation. He was not saying that to the wicked, but to His Church. It also explains why He said in Rev.3:4 there were "... a few names even in Sardis which have not defiled their garments; and they shall walk with Me in white: for they are worthy."