The shepherd of the sheep is the one who is worthily endowed with the gift of teaching and uses the lawful entrance. In other words, he is the one who endeavors to live entirely according to the teaching of the law and thus enters the sheepfold exactly as he should. He then leads all the rest, like sheep, to the pastures of his teaching by showing them the food of his words, with which they must first nourish themselves. Once they have done so, he explains the power of those words and how they must be understood. He also points out to the sheep those teachings that others might deceitfully propose to them for their perdition but from which they should abstain.
Since this is what the shepherd is like, it follows that a thief and a bandit is the exact opposite. He does not use the lawful entrance, nor does he show respect for the precepts of the law. This is how he teaches the people given to him: he simply scales the fence and seizes the entrance as well as the office of teacher, because he has done nothing that would entitle him to such honor. He behaves inconsiderately and, by doing whatever he wants, does things that harm the sheep. Indeed, how can one who does not discipline himself in the precepts of the law be useful to anyone else? In other words, the Lord is saying, “Let us discern between you and me, if you like, who uses the lawful entrance; who diligently follows the precepts of the law; for whom Moses, the gatekeeper of the sheepfold, opens the gate; to whom offers praise for completing his work and who he considers worthy to be declared shepherd because of what he has done.…
Then what does he say? I am the gate; that is, “I am not like them; they are thieves. I am the true door.” If anyone enters by me, he will be saved, and will go in and out and find pasture. Those who come to me will receive truth through me. They will enjoy true life and safely graze in the truth, being filled with delight by the teaching of the divine law.” The words, he will go in and out and find pasture, refer to what happens among sheep that derive great delight from his doctrine of truth.
But how can it be ascertained that they are the thieves and you are the shepherd of the sheep? Since what he had said could not be verified by words only, his words also had to be confirmed by evidence. And so He added, The thief comes only to steal and kill and destroy; I came that they may have life, and have it abundantly. He confirms His argument by citing those actions that are typical of thieves. “Thieves exist to steal, kill and destroy for their own greed. I am so far removed from these actions that I, in fact, do the exact opposite for those who obey me. In other words, I give them life—and not just any life, but eternal life.” This is what he meant when he said, and have it abundantly, alluding to the resurrection he will give to the human race.
Theodore of Mopsuestia, Commentary on John 4.10