Did he believe that it was possible for a believer to fall away, and perish forever? We know he did not. Let us read what Scripture does say on this matter. We read Paul's own inspired words, " 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." (1 Cor. 9:2727But 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. (1 Corinthians 9:27)).
Note carefully what the Apostle did not say. He did not say, Lest after I was SAVED. But no, he said, " Lest when I have PREACHED to others." Salvation is a gift of God never to be recalled. Preaching is a matter of profession. Happy is the servant of the Lord, whose profession is real. Judas Iscariot preached, but he was never saved, and all the time a traitor at heart. Preaching may not always be accompanied by reality. Too often it is not, as we know full well today. The Apostle Paul, however, proved his reality by keeping his body under, bringing it into subjection. If a man claims to be the Lord's, and makes no effort to refuse the flesh within him, he would clearly be no child of God.
The word, castaway, is used for a counterfeit coin. Now a gold coin is a gold coin, and can be nothing else. A counterfeit coin is a base coin, and can be nothing else. But Paul, a real servant of God, was true gold, and could not be found to be anything else; just as Judas Iscariot was a base coin, and could not be found to be anything else. Paul did not say he might become a castaway, but be a castaway, that is false all the time. Of himself he wrote with assurance, "I therefore so run, NOT AS UNCERTAINLY; so fight I, NOT AS ONE THAT BEATETH THE AIR." (1 Cor. 9:2626I therefore so run, not as uncertainly; so fight I, not as one that beateth the air: (1 Corinthians 9:26)). There was no uncertainty about him. He was a real servant of the Lord. He knew he was saved. He knew he was no counterfeit. He knew on whom he had believed, and was persuaded that what he had committed to Christ was safe in His keeping till the day of glory (2 Tim. 1:1212For the which cause I also suffer these things: nevertheless I am not ashamed: for I know whom I have believed, and am persuaded that he is able to keep that which I have committed unto him against that day. (2 Timothy 1:12)).
It would be well at this point to consider the difference between backsliding and apostasy.