I like you thought that "not doing something isn’t a work" i.e. Repentance is not a work, but rather putting off the "works of the flesh" (Gal 5:19). I'm going to have to think more on that idea.
A truly saved person WILL ongoingly repent from sin and grow in the grace of God (and it may feel like work) but that is part of the result of salvation and nothing which we can claim attributed to our salvation.
We are commanded work to do once we’re saved - 1 Corinthians 15:58 “Therefore, my beloved brethren, be ye stedfast, unmoveable, always abounding in the work of the Lord, forasmuch as ye know that your labour is not in vain in the Lord.” But this is the work which shows our faith and a result of salvation, not contributing to it.
But the fact of Paul’s many w@rnings to the brethren about not being deceived about living a sinful lifestyle as a believer, because if they do they won’t make it to heaven, is ample proof of the believers daily choice to walk in the light, or walk in the flesh.
Paul even stated he has to subjugate his flesh daily, lest he become a reprobate, and warned the brethren to examine themselves, whether they are in the faith, or are reprobates.
2Co 13:5 Examine yourselves, whether ye be in the faith; prove your own selves. Know ye not your own selves, how that Jesus Christ is in you, except ye be reprobates?
He warns that some of the church that think they’re in the faith can actually have become reprobates.
The word reprobates is ADOKIMOS in the Greek, which becomes important in the verse where Paul himself says he has to subjugate his flesh daily, lest after preaching salvation to others, he end up a castaway.
1Co 9:27 But I keep under my body, and bring itinto subjection: lest that by any means, when I have preached to others, I myself should be a CASTAWAY.
The word castaway is also ADOKIMOS in the Greek - reprobate - castaway and rejected by God.
G96 (Strong)
ἀδόκιμος
adokimos
ad-ok'-ee-mos
From G1 (as a negative particle) and G1384; unapproved, that is, rejected; by implication worthless(literally or morally): - castaway, rejected, reprobate.
Total KJV occurrences: 8
Paul said he could end up a reprobate if he didn’t subjugate his flesh with its sinful desires, daily.
Albert Barnes exegetes the verse:
“The simple idea of Paul is, that he was afraid that he should be disapproved, rejected, cast off; that it would appear, after all, that he had no religion, and would then be cast away as unfit to enter into heaven”.
John Wesley also exegetes 1 Corinthians 9:27:
This single text may give us a just notion of the scriptural doctrine of election and reprobation; and clearly shows us, that particular persons are not in holy writ represented as elected absolutely and unconditionally to eternal life, or predestinated absolutely and unconditionally to eternal death; but that believers in general are elected to enjoy the Christian privileges on earth; which if they abuse, those very elect persons will become reprobate. St. Paul was certainly an elect person, if ever there was one; and yet he declares it was possible he himself might become a reprobate. Nay, he actually would have become such, if he had not thus kept his body under, even though he had been so long an elect person, a Christian, and an apostle.
Paul never taught unconditional eternal security - in fact he taught the opposite.
Paul said he could end up a reprobate if he didn’t subjugate his flesh with its sinful desires, daily.