In the verse where Paul says he could be disqualified, is he referring to being disqualified from having a home in Heaven, or from ministry? Some writers and commentaries think when he refers to being disqualified, that he’s concerned about being disqualified from doing ministry work, instead of being disqualified from salvation.
"I myself should be a castaway,
or rejected, or disapproved of; that is, by men: the apostle's concern is, lest he should do anything that might bring a reproach on the Gospel; lest some corruption of his nature or other should break out, and thereby his ministry be justly blamed, and be brought under contempt; and so he be rejected and disapproved of by men, and become useless as a preacher: not that he feared he should become a reprobate, as the word is opposed to an elect person; or that he should be a castaway eternally, or be everlastingly damned; for he knew in whom he had believed, and was persuaded of his interest in the love of God, and that he was a chosen vessel of salvation, that could not be eternally lost: though supposing that this is his sense, and these his fears and concern, it follows not as neither that he was, so neither that he could be a lost and damned person: the fears of the saints, their godly jealousies of themselves, and pious care that they be not lost, are not at all inconsistent with the firmness of their election, their security in Christ, and the impossibility of their final and total falling away; but on the contrary are overruled, and made use of by the Spirit of God, for their final perseverance in grace and holiness." [John Gill's Exposition of the Bible].
"The point in the entire context is not about receiving eternal life or going to heaven when Paul dies, but is instead about being faithful and useful to God in what God wants to accomplish in Paul’s life. The prize is not eternal life, for eternal life is a free gift of God.
The prize, or the crown, is significance in the Kingdom of God, blessing in his life and ministry, and praise from God when he stands before Him for a life well-lived in His service.
So Paul is not concerned with losing his eternal life, but is very much concerned with being disqualified for ministry.
Since Paul desires to continue his ministry, and to run in a way that is pleasing and honoring to God, he runs with care and perseverance." [Jeremy Myers, "Does 1 Corinthians 9:27 teach that you can lose your salvation?"]
"Therefore, beloved, since you look for these things, be diligent to be found by Him in peace, spotless and blameless, and regard the patience of our Lord as salvation; just as also our beloved brother Paul, according to the wisdom given him, wrote to you, as also in all his letters, speaking in them of these things, in which are some things hard to understand, which the untaught and unstable distort, as they do also the rest of the Scriptures, to their own destruction" (II Peter 3:14-16).
You can always find people who will twist Scriptures to make it fit their notions of what they think God meant to say. Both authors make elaborate arguments regarding the word adokimos, which does mean to be rejected, depraved, disqualified, unworthy, or useless. What they argue is that the word can't be applied to salvation because everyone knows that salvation cannot be lost so they look for something else that Paul might fear losing.
Yet, adokimos is used in the following passages:
"Test yourselves to see if you are in the faith; examine yourselves! Or do you not recognize this about yourselves, that Jesus Christ is in you -- unless indeed you fail the test? But I trust that you will realize that we ourselves do not fail the test" (II Corinthians 13:5-6).
"Just as Jannes and Jambres opposed Moses, so these men also oppose the truth, men of depraved mind, rejected in regard to the faith" (II Timothy 3:8).
"For ground that drinks the rain which often falls on it and brings forth vegetation useful to those for whose sake it is also tilled, receives a blessing from God; but if it yields thorns and thistles, it is worthless and close to being cursed, and it ends up being burned" (Hebrews 6:7-8).
These passages show that adokimos can be used in regards to a loss of salvation.
"For though I am free from all men, I have made myself a slave to all, so that I may win more. To the Jews I became as a Jew, so that I might win Jews; to those who are under the Law, as under the Law though not being myself under the Law, so that I might win those who are under the Law; to those who are without law, as without law, though not being without the law of God but under the law of Christ, so that I might win those who are without law. To the weak I became weak, that I might win the weak; I have become all things to all men, so that I may by all means save some. I do all things for the sake of the gospel, so that I may become a fellow partaker of it.
Do you not know that those who run in a race all run, but only one receives the prize? Run in such a way that you may win. Everyone who competes in the games exercises self-control in all things. They then do it to receive a perishable wreath, but we an imperishable. Therefore I run in such a way, as not without aim; I box in such a way, as not beating the air; but I discipline my body and make it my slave, so that, after I have preached to others, I myself will not be disqualified" (I Corinthians 9:19-27).
Often the way to understand a passage is to look at its greater context. Paul was discussing teaching others and notice that I Corinthians 9:23 and I Corinthians 9:27 are similar points -- one stated positively and the other negatively. Paul's focus in teaching was to save people from destruction and in the process, Paul was also aiming to be saved himself.
While life continued, Paul knew that he might lose his way so he was determined to remain faithful. "Not that I have already obtained it or have already become perfect, but I press on so that I may lay hold of that for which also I was laid hold of by Christ Jesus. Brethren, I do not regard myself as having laid hold of it yet; but one thing I do: forgetting what lies behind and reaching forward to what lies ahead, I press on toward the goal for the prize of the upward call of God in Christ Jesus" (Philippians 3:12-14).