A lot of this would go against Paul's writings in Romans and other places that indicate the believer is saved and does not lose their salvation.
That is correct, for it, and its companion teachings such as Hebrews 10:25-39, is referring to having "wickedly departed" from God, which David could say he did not do, 2 Samuel 22:22) and prayed God would keep Him from the "great transgression." Ps. 19:13)
Such are those who are often reproved, but impenitently continue in will-full sin, or like Judas, make a definite will-full decision to recant the faith they once definitely will-fully professed, which is in contrast to those who struggle with a besetting sin, (Heb. 12:1) or sin even as David did, somehow subduing his conscience, but repent with "godly sorrow" when convicted of God, as David did. which often takes chastisement. (2 Corinthians 7:9,10; 1Co. 11:32)
A lot of this would go against Paul's writings in Romans and other places that indicate the believer is saved and does not lose their salvation.
The believer is indeed saved and does not lose their salvation, but are warned as believers against denying the faith, (1Tim. 5:8) and effectively becoming unbelievers, of "departing from the living God," (Heb. 3:12) and drawing back to perdition, (Heb. 10:38,39) making Christ to profit them nothing, of no effect, being fallen from the grace whereby Christ had set them free. (Galatians 5:1-5)
As effectual faith in conversion appropriated justification by grace, so denying the faith in recanting like as he once truly confessed it forfeits that status. Salvation is by faith, but by grace one is motivated and enabled to believe, and God then continues to work to motivate and enable us to obey Him, and thus we cannot really claim any merit, to the glory of God!
But we can resist God, (Prov. 1:20-32) and for which is what we must take credit for.
OSAS proponents typically attempt to argue that such verses as cited above were not addressed to believers, but which is manifestly contextually false, and we are to go wherever the Truth leads.
I would like to be able to say that the warning as hypothetical, but I do not see that as tenable.