The preceding passage (from v. 26) and the similar
passage, vi. 6-6, have proved perplexing to many readers; and were such a
stumbling-block to Luther, that they caused him even to deny the canonical
authority of the Epistle. Yet neither the passage asserts the impossibility
of an apostate’s repentance. What is said amounts to this—that for the
conversion of a deliberate apostate, God has (according to the ordinary laws of
His working) no further means in store than those which have been already tried
in vain. It should be remembered, also, that the parties addressed are not
those who had already apostatized, but those who were in danger of so doing,
and who needed the most earnest warning. (W. J. Conybeare and J. S. Howson, The
Life and Epistles of St. Paul [Grand Rapids, Mich.: Eerdmans, 1964], 809 n.
9)