You are mistaken on exactly what the doctrine is. Perhaps others may have a perverted view on what the doctrine is and you have discussed the topic, but I assure you it is not what you believe it to be.
Before you get ahead of yourself, let's find out...
The perseverance of the saints means that all those who are truly born again will be kept by God’s power and will persevere as Christians until the end of their lives, and that only those who persevere until the end have been truly born again.
Wayne Grudem from Systematic Theology (pg. 788)
That's pointing to the concept of Calvinism, that those saved are under predesinted fate, and not under their own free will. It is only an excuse to declare that one who once believed falls away from Christ was never... a real believer on Him in the first place. Once again, that is the idea of fate, and not the responsibility of the individual to make a 'choice'.
Further, the idea of persevering there is being used against the idea of the next quote...
If our religion be of our own getting or making, it will perish; and the sooner it goes, the better; but if our religion is a matter of God's giving, we know that He shall never take back what He gives, and that, if He has commenced to work in us by His grace, He will never leave it unfinished.
C.H. Spurgeon
Though I don't like his terminology, I get his point, but he's also pointing to predestined fate and not free will choice. It is free will choice by which one must accept Christ, and it is free willl choice for a believer that later decides to reject Him. Spurgeon's idea leaves no room for the latter, except only to say Christ's grace never was at work with the one who later fell away.
The fact that our Lord Jesus said all manner of sin can be forgiven of men, except for the unpardonable sin, Spurgeon leaves NO room even for the existence of the unpardonable sin, a sin which points to a believer that once tasted of the gifts and power of the world to come through Christ Jesus, but then rejects Him later (Hebrews 5 & 6).
For non-reformed theologies..."at the end of the day, the security of the believer finally rests with the believer. For those in the opposite camp [Reformed], the security of the believer finally rests with God -- and that, I suggest, rightly taught, draws the believer back to God himself, to trust in God, to a renewed faith that is of a piece with trusting him in the first place."
D.A. Carson
Throw away his argument of non-reformed vs reformed; that idea is irrelevant. Carson likewise is saying the same things as the previous quotes, i.e., Salvation is assured because it rest with God, and not with the believer. Once again that's Calvinism's idea of predestination fate, i.e., a true believer won't fall away because God won't allow it. That's NOT what God's Word teaches at all though.
It's these kind of concepts that cause the erroneous idea that Faith and actions by the believer under God's Grace has no effect upon their salvation. It's what leads to the false idea that one within this fold can no longer be subject to sin. That has lead in later years to the preaching that these cannot sin, and thus the Once Saved Aways Saved doctrine of men
Am I showing that I don't understand the OSAS doctrine? You'd be very wrong if you thought so.
I don't know if you will understand the following. But I'm going to say it anyway.
Apostle Paul spoke of the idea of predestination especially within the Romans 8 chapter which is on a different type admonition than many of his other admonitions to the Churches. In many of his Epistles he is continually warning brethren to not sin, warning them to remain focused in Christ, work out their own salvation, etc., and especially beware of allowing any man to deceive them.
Now if Apostle Paul's teaching truly aligned with the Calvinist traditions of those above quotes, then Paul would have had no need to do such admonishing of the brethren to stay in Christ and not fall away like he did.
But notice how the Rom.8 Scripture reads differently...
Rom 8:28-31
28 And we know that all things work together for good to them that love God, to them who are the called according to His purpose.
29 For whom He did foreknow, He also did predestinate to be conformed to the image of His Son, that He might be the firstborn among many brethren.
30 Moreover whom He did predestinate, them He also called: and whom He called, them He also justified: and whom He justified, them He also glorified.
31 What shall we then say to these things? If God be for us, who can be against us?
(KJV)
Well, did even Apostle Paul forget about Christ's Own Words about the unpardonable sin? Did Paul also forget about Christ saying many are called but few are chosen? Did Paul also forget about our Lord's warnings about the five foolish virgins, and those of Matt.7 that cast out devils in His Name and have many wonderful works in His Name, but Christ tells to get away from Him when He comes?
Here Paul is getting ready to leave the brethren, and he warns them to take heed, because after his departing he knew wolves would come in not sparing the flock.
Acts 20:27-31
27 For I have not shunned to declare unto you all the counsel of God.
28 Take heed therefore unto yourselves, and to all the flock, over the which the Holy Ghost hath made you overseers, to feed the church of God, which he hath purchased with his own blood.
29 For I know this, that after my departing shall grievous wolves enter in among you, not sparing the flock.
30 Also of your own selves shall men arise, speaking perverse things, to draw away disciples after them.
31 Therefore watch, and remember, that by the space of three years I ceased not to warn every one night and day with tears.
(KJV)
In his Epistle to Timothy, Paul also warns how the time would come when some would not endure sound doctrine and turn away from the truth (2 Tim.4).
Does that mean those believers never were actually offered true Salvation through Christ Jesus? No, of course not.
The difference is spelled out in John 17 with what our Lord Jesus prayed. Revealed there are two groups, one being His elect 'chosen' servants The Father owned and gave to Christ. The other group are those who would come to believe on Christ Jesus through His elect's word (i.e., their preaching of The Gospel). The first group, like Christ's Apostles, are 'called' and... 'chosen', and the second group of believers are 'called' only.
I assure you, Apostle Paul understood this distinction between two groups of believers on our Lord Jesus. When Paul was continually admonishing the brethren in the Churches to watch and remain spritiually sober, he knew he was speaking to those 'called' only that can... by their own free will... choose to fall away from Christ. That's why those need continual admonishing to stay in Christ Jesus in order to become joined with His elect 'chosen'.
Christ's chosen sent elect cannot... be swayed to fall away from Christ Jesus.
John 17:5-10
5 And now, O Father, glorify Thou Me with Thine Own self with the glory which I had with Thee before the world was.
6 I have manifested Thy name unto the men which Thou gavest Me out of the world: Thine they were, and Thou gavest them Me; and they have kept Thy word.
7 Now they have known that all things whatsoever Thou hast given Me are of Thee.
8 For I have given unto them the words which Thou gavest Me; and they have received them, and have known surely that I came out from Thee, and they have believed that Thou didst send Me.
9 I pray for them: I pray not for the world, but for them which Thou hast given Me; for they are Thine.
10 And all Mine are Thine, and Thine are Mine; and I am glorified in them.
(KJV)
That's ownership, about Christ's elect that belonged to The Father Who gave them to Christ. Apostle Paul was one of those. The example of Christ intervening directly with Paul to put him in His service is an example of this ownership of His elect 'chosen' sent ones.
Those are especially who the Romans 8 Scripture is about, the truly predestinated, sanctified, and glorified, the chosen elect.
But here's about the rest of us who are 'called' only...
John 17:18-21
18 As Thou hast sent Me into the world, even so have I also sent them into the world.
19 And for their sakes I sanctify Myself, that they also might be sanctified through the truth.
20 Neither pray I for these alone, but for them also which shall believe on Me through their word;
21 That they all may be one; as Thou, Father, art in Me, and I in Thee, that they also may be one in Us: that the world may believe that Thou hast sent Me.
(KJV)
The meaing of the word apostle from the Greek means to be 'sent'. Christ's 'chosen' elect are 'sent' by Him into this world, even as The Father 'sent' Christ into the world. That's about ownership again.
Those of us who come to believe on Jesus through the word of His elect Apostles are 'called', not 'sent', but 'called' only. We must... by our OWN free will... seek to stay in Christ Jesus. We CAN still fall away from Christ IF we are not careful.
But what Calvin, et al did was to take Scripture like Romans 8 which is especially about the chosen elect of Christ that can NEVER fall away, they took that idea and wrongly began applying it to the whole Church on earth, making it as if it were impossible for a believer to lose their salvation, nor able to fall away from our Lord Jesus Christ. Yet Christ's Apostles continually warned brethren about abiding in fleshy lusts and sin to their destruction if they didn't repent and make a change. That is how the OSAS doctrine started.
