27. My sheep hear my voice—As the blind-born did. Those who are bent on holiness and salvation show it by listening to Christ and his Gospel. It is very illogical to infer from all this the doctrine that no man will lose or abandon the character of a sheep of Christ, that is, of a true believer. Qualities or conduct ascribed to persons as possessed of a certain character, do not imply that the character itself may not change. A hireling fleeth because he is a hireling; but that does not prove but that the man may cease to be a hireling. A thief and a robber will kill and destroy; but that does not prove that a thief or robber may not, like Saul of Tarsus, cease to be a thief and robber. So a sheep will follow Christ; but that does not imply but that the man may cease to be a sheep and even become a goat. For a man may as truly from a sheep become a goat, as from a goat become a sheep.
28. Shall never perish—No sheep of Christ can ever perish. The unbeliever and the apostate will perish, but neither the unbeliever or the apostate is a sheep of Christ. Perish… pluck—The literal sheep of the human shepherd may perish by the robber, or be plucked away by the thief; but the spiritual sheep of the true Shepherd no robber can kill, no thief can steal. He must by his own free act abandon or forfeit his spiritual character before he can lose his eternal privilege. (Daniel Whedon, Commentary, pg. 376, Wesley Heritage Collection CD)