I think there have been good articles written on the issue of preservation of the saints on other websites. I think it would be good to gather a few passages that don’t often make the list here.

http://triablogue.blogspot.com/2019/02/can-christian-lose-salvation.html

2 Peter 2:19-22

¹⁹ They promise them freedom, but they themselves are slaves of corruption. For whatever overcomes a person, to that he is enslaved. ²⁰ For if, after they have escaped the defilements of the world through the knowledge of our Lord and Savior Jesus Christ, they are again entangled in them and overcome, the last state has become worse for them than the first. ²¹ For it would have been better for them never to have known the way of righteousness than after knowing it to turn back from the holy commandment delivered to them. ²² What the true proverb says has happened to them: “The dog returns to its own vomit, and the sow, after washing herself, returns to wallow in the mire.”

There is a debate as to whether this refers to recent Christian converts or to false teachers. In fact, it could be about both and not affect the argument from the person maintaining that they are losing their salvation.  I think the reason it is convincing to think it is about actual believers is that these things that sound like believers. Namely, they had “knowledge of our Lord and Savior Jesus Christ” and that they “ turn back from the holy commandment”.

I think the reason to doubt this is because in Petrine categories to have escaped the defilements of the world is how he presents conversion. 2 Peter 1:3-4 states:

³ His divine power has granted to us all things that pertain to life and godliness, through the knowledge of him who called us to his own glory and excellence, ⁴ by which he has granted to us his precious and very great promises, so that through them you may become partakers of the divine nature, having escaped from the corruption that is in the world because of sinful desire.

The truth is apparent to me that the individuals in chapter 2 have not fulfilled this. They are rather “dogs”, “swine”, etc unclean and have remained in their worldly desires rather than remaining in the knowledge of the Lord given to them when they were pretending Christians in the Church. In fact, if you hold to the authenticity of 2 Peter you should also hold to 1 Peter which states:

³ Blessed be the God and Father of our Lord Jesus Christ! According to his great mercy, he has caused us to be born again to a living hope through the resurrection of Jesus Christ from the dead, ⁴ to an inheritance that is imperishable, undefiled, and unfading, kept in heaven for you, ⁵ who by God’s power are being guarded through faith for a salvation ready to be revealed in the last time. 

This suggests that God has promised to guard Christian until their salvation is to be revealed. Peter assures his readers that they will certainly receive this inheritance. That future salvation or eschatological deliverance will be theirs. The reason for this confidence is that they “are guarded by God’s power.” That is to say, God has promised to preserve the faithful. So, in order for his promise to not occur, is for God to fail to be truthful to his word. Dr. Thomas Schreiner presents an explanation:

The best solution is to say that 2 Peter uses phenomenological language. In other words, Peter uses the language of “Christians” to describe those who fell away because they gave every appearance of being Christians. They confessed Christ as Lord and Savior, were baptized, and joined the church. But the false teachers and some of those they seduced, though still present physically in the church, were no longer considered to be genuine believers by Peter. Nonetheless, he used “Christian” language to describe them, precisely because of their participation in the church, because they gave some evidence initially of genuine faith. Those who had apostatized revealed that they were never truly part of the people of God since remaining true to the faith is one sign that one truly belongs to God. The words of 1 John apply well to what has happened in 2 Peter: “They went out from us, but they did not belong to us; for if they had belonged to us, they would have remained with us. However, they went out so that it might be made clear that none of them belongs to us” (1 John 2: 19). Peter points in the same direction in the illustration of the dog and pig. In the final analysis, those who fell away never really changed their nature. 1767 They remained dogs and pigs inside. 1768 As Hafemann argues at some length their nature was the cause of their actions. 1769 They may have washed up on the outside and appeared to be different, but fundamentally they were dogs and pigs. In other words, they were always unclean; they only seemed to have changed. Perseverance, therefore, is the test of authenticity.

Schreiner, Thomas R.. 1-2 Peter and Jude: The Christian Standard Commentary (Kindle Locations 9167-9180). B&H Publishing Group. Kindle Edition.

 

James 5:19-20

¹⁹ My brothers, if anyone among you wanders from the truth and someone brings him back, ²⁰ 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.

In this, we have James discussing the issue of those drifting away from the faith. James leaves it ambiguous whether he is saved and abandons the faith (phenomenological Christian) to later return to the teachings and accept it or someone that has believed (backsliding) and has ventured from the faith only to return to it and receive eschatological salvation. As one scholar noted:

 

Meanwhile, the soul of the person who did the restoring would not be saved by this action, as we would assume that such a person already does believe, thus making them want to restore the wanderer. This restoring is more a work that shows their faith than one that creates their salvation. The implication of “saves their soul” may well be that the wanderer never truly believed, and it is in this restoration that they come to their own saving faith. Or James may be using “saved” here in the sense of final, eschatological salvation — they have now been brought safely to the end of the process that their earlier belief initiated.

Blomberg, Craig L.; Kamell, Mariam J.. James (Zondervan Exegetical Commentary on the New Testament) (Kindle Locations 7537-7542). Zondervan. Kindle Edition.

The other issue is a question of who’s sins are going to be covered. Most think it is the person that has fallen away, but some scholars think because Jewish sources usually posit it is the one that returns the one that strays needs the forgiveness. Another alternative is that it is ambiguous because we all are “in need of God’s grace, all “prone to wander” given the opportunity and right inducement. Thus, in this one act of righteousness in turning another person who has wandered farther astray, we find ourselves drawn back closer to God’s grace and righteousness.” This is also from Blomberg and I think it is fitting.