John 10:27 My sheep hear my voice, and I know them, and they follow me:
Sorry, but a lot of Calvinists fail to understand that someone who falls away from Christ, is backslidden, does NOT & is NOT following Christ. Ergo...they are NOT His sheep.
1 Corinthians 9:27 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.
Sorry again, but it seems even Calvinist exegetes agree that the Apostle Paul feared becoming a reprobate after "preaching to others..." Let's look at the 'prince of preachers, Charles Spurgeon has to say -
Castaway = rejected, a reprobate.
I myself should become a reprobate - Disapproved by the Judge, and so falling short of the prize. 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.
St. Paul contemplates with horror the possibility of his preaching the gospel to others, and, by reason of his personal inconsistencies, proving at last a "castaway." No amount of religious profession, no fervent in religious work, no mere utterance of religious sentiment, can avail without personal and practical consistency of life.
Or, from Albert Barnes, another Calvinist -
"...Paul is afraid that he should be disapproved, rejected, cast off; that is would appear, after all, that he had no religion, and would then be cast away as unfit to enter into heaven...Ministers, like others, are in danger of losing their souls. If Paul felt this danger, who, is there among the ministers of the Cross who should not feel it? If Paul was not safe, who is? The fact that a man has preached to many is no certain evidence that he will be saved, Paul had preached to thousands and yet he felt that after all this there was a possibility that he might be lost.
Or, from John Wesley, not a Calvinist -
I myself should become a reprobate - Disapproved by the Judge, and so falling short of the prize. 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.
Or, from Adam Clarke, not a Calvinist -
On the subject of the possibility of St. Paul becoming a castaway, much has been said in contradiction to his own words. He most absolutely states the possibility of the case: and who has a right to call this in question?
So...i'm going to defer to men of God who knew Koine Greek as well as their native tongue & agree that one can definitely give up in the race & become a castaway - rejected by God, a reprobate doomed to eternal destruction!