1 Corinthians 9:27
27 But I keep under my body, and bring it into subjection: lest that by any means, when I have preached to others, I myself should be a castaway.
This verse is very often used to teach that even the Apostle Paul was afraid of losing his Salvation, and so certainly, we should be too.
Such see the mention of being "a castaway" as a reference to loss of Salvation. Is this a valid interpretation?
The entire chapter of 1 Corinthians 9 is the Apostle Paul speaking about his ministry.
The first section of the chapter has to do with the fact that preachers have a right to receive wages for their preaching, but Paul himself did not take advantage of this (1 Corinthians 9:1-19).
He then discusses his ministry more generally, and this is where the passage in question is located:
1 Corinthians 9:20-27
20 And unto the Jews I became as a Jew, that I might gain the Jews; to them that are under the law, as under the law, that I might gain them that are under the law;
21 To them that are without law, as without law, (being not without law to God, but under the law to Christ,) that I might gain them that are without law.
22 To the weak became I as weak, that I might gain the weak: I am made all things to all men, that I might by all means save some.
23 And this I do for the gospel's sake, that I might be partaker thereof with you.
24 Know ye not that they which run in a race run all, but one receiveth the prize? So run, that ye may obtain.
25 And every man that striveth for the mastery is temperate in all things. Now they do it to obtain a corruptible crown; but we an incorruptible.
26 I therefore so run, not as uncertainly; so fight I, not as one that beateth the air:
27 But I keep under my body, and bring it into subjection: lest that by any means, when I have preached to others, I myself should be a castaway.
In context, we can see that he's discussing his approval before the Lord when He examines his ministry. He's worried about losing his opportunity to minister, and wants to prevent himself from becoming careless or complacent in his ministry.
We see similar thoughts from the Apostle Paul elsewhere in the Epistles, where he is worried about the effectiveness and fruit of his ministry:
Galatians 4:11
11 I am afraid of you, lest I have bestowed upon you labour in vain.
Philippians 2:16
16 Holding forth the word of life; that I may rejoice in the day of Christ, that I have not run in vain, neither laboured in vain.
Naturally, he views his ministry as the most import thing in his life, and this comes out in his writing.
Personal Salvation, on the other hand, and the fear that he might lose that, is nowhere to be found in the context. We already know from many other verses that the Apostle Paul had absolute assurance that he was going to Heaven when he died:
2 Timothy 4:18
18 And the Lord shall deliver me from every evil work, and will preserve me unto his heavenly kingdom: to whom be glory for ever and ever. Amen.
Romans 8:38-39
38 For I am persuaded, that neither death, nor life, nor angels, nor principalities, nor powers, nor things present, nor things to come,
39 Nor height, nor depth, nor any other creature, shall be able to separate us from the love of God, which is in Christ Jesus our Lord.
Philippians 1:23
23 For I am in a strait betwixt two, having a desire to depart, and to be with Christ; which is far better:
He had assurance of Salvation, because he rightly understood, and consistently taught in all of his writings, that Salvation was a "free gift" (Romans 5:15-18) obtained by faith alone (Ephesians 2:8-9).
This verse has nothing to do with loss of Salvation. The only way to arrive at that conclusion is to lift it completely out of the context, and ignore dozens of other things that the Apostle Paul wrote.
Instead, it teaches that even the greatest preacher of all time had anxieties about, and took precautions against, being disqualified from his ministry, and opportunity to serve God here on earth.