What is Perseverance of the Saints?
Perseverance of the Saints can also be referred to as "Eternal Security", and is often summed up as "Once Saved Always Saved" (though in my opinion, this is far too simplistic, so I rarely use it, and would advise you don't either). Put simply, it states that once salvation has been granted, it cannot be lost. Even apostasy, for the true believer, is impossible. The Arminian equivalent would be Conditional Security; the idea that salvation can somehow be lost, whether through some sin, or through the loss of faith.
The Bible Brain Position
For sake of consistency, I have maintained the subheading "The Bible Brain position", but I could just as easily have labelled this "The Biblical Position", because of all of Calvinism's 5 points, I see Perseverance of the Saints as being nigh unassailable. I wholeheartedly agree with it, and though I basically always have believed it, I didn't know it was part of Calvinism for quite some time. Conditional Security just does not work. The key to understanding Perseverance of the Saints is to understand how, exactly, salvation works. There are only two sensible ways one could ever be said to lose salvation: Either you did too much evil/not enough good to be saved, or you apostatised. In all other scenarios, a Christian is saved. To lose one's salvation by works is, of course, Biblically impossible. Salvation is received by grace of God, through faith in His Son, not of works, but as a gift from God (see Ephesians 2:8-10). Because of this, no amount of good works is required for salvation, neither is any amount of evil sufficient to lose it. The idea of salvation is that Christians and Christ effectively "swap verdicts". On the cross, Christ became sin, and so we who believe become the righteousness of God in Him (2 Corinthians 5:21). Thus, there is no way for those who believe to work their way out of salvation, just as there is no way to work your way into it. But what about apostasy? Evidently, one cannot remain saved if he apostatises. Can an apostate not, therefore, be said to have lost salvation? John answers that in 1 John 2:19, when he says "They went out from us, but they were not of us; for if they had been of us, they would no doubt have continued with us: but they went out, that they might be made manifest that they were not all of us." As should be abundantly clear to even the most hard-hearted, 5 point Arminian, it is entirely possible to profess faith, but not actually have it. This, according to John, is not only the case with apostates, but their apostasy was actually intended to reveal that this was the case. If they were truly of us (i.e., if they were truly saved), they would have remained saved. Because they left, they prove they weren't. This is because of the indwelling of the Holy Spirit. When a person gets saved, they receive the Holy Spirit, who, according to scripture, seals us as a guarantee of eternal life (2 Corinthians 1:22; Ephesians 1:13, 4:30). He is perfectly able to do this (Jude 1:24), and, when paired with John 10:28-29, we actually see that the entirety of the Godhead is working to ensure that a true believer cannot lose their salvation. When you become a Christian, you are a Christian for life. Thus, we see that of all the doctrines of Calvinism, Perseverance of the Saints is by far the most powerful. I dare even say it is nigh irrefutable.
These things have I written unto you that believe on the name of the Son of God; that ye may know that ye have eternal life, and that ye may believe on the name of the Son of God.
- 1 John 5:13 KJV