Some people use the above verse to prove that even a true Christian can be lost. This is a mistake, as we know from many passages of Scripture (John 10:28-3028And I give unto them eternal life; and they shall never perish, neither shall any man pluck them out of my hand. 29My Father, which gave them me, is greater than all; and no man is able to pluck them out of my Father's hand. 30I and my Father are one. (John 10:28‑30), etc.). The word "castaway," however, must not be taken in a limited sense, as though it meant merely disapproved in his ministry. It is not so used in the New Testament.
What makes the difficulty in some minds is, that they fail to see that it is not here a question of a true Christian's being a castaway, but of his being a preacher and a castaway. Alas, we know there are many such, and Paul wanted to show by his whole mode of life, in which he kept his body under, that he was not one of these.
He says in chapter 4:6, "And these things, brethren, I have in a figure transferred to myself and to Apollos for your sakes"; this explains the words in our verse, "I myself." It was not that Paul himself could be lost or cast away, but he applies it to himself as a preacher to make the case plain.