1 Our Lord's discourse to the Jews began in connection with the man who was born blind and was restored to sight. Your Charity therefore ought to know and be advised that today's lesson is interwoven with that one. For when the Lord had said, “For judgment I have come into this world; that they who see not might see, and they who see might be made blind,”— which, on the occasion of its reading, we expounded according to our ability—some of the Pharisees said, “Are we blind also?” To whom He replied, “If you were blind, you should have no sin: but now ye say, We see; [therefore] your sin remains.” To these words He added what we have been hearing today when the lesson was read.
2. “Verily, verily, I say unto you, He that enters not by the door into the sheepfold, but climbs up some other way, the same is a thief and a robber.” For they declared that they were not blind; yet could they see only by being the sheep of Christ. Whence claimed they possession of the light, who were acting as thieves against the day? Because, then, of their vain and proud and incurable arrogance, did the Lord Jesus subjoin these words, wherein He has given us also salutary lessons, if we lay them to heart. For there are many who, according to a custom of this life, are called good people—good men, good women, innocent, and observers as it were of what is commanded in the law; paying respect to their parents, abstaining from adultery, doing no murder, committing no theft, giving no false witness against any one, and observing all else that the law requires— yet are not Christians; and for the most part ask boastfully, like these men, “Are we blind also?” But just because all these things that they do, and know not to what end they should have reference, they do to no purpose, the Lord has set forth in today's lesson the similitude of His own flock, and of the door that leads into the sheepfold. Pagans may say, then, We live well. If they enter not by the door, what good will that do them, whereof they boast? For to this end ought good living to benefit every one, that it may be given him to live for ever: for to whomsoever eternal life is not given, of what benefit is the living well? For they ought not to be spoken of as even living well, who either from blindness know not the end of a right life, or in their pride despise it. But no one has the true and certain hope of living always, unless he know the life, that it is Christ; and enter by the gate into the sheepfold.
Source: Tractates on the Gospel of John (New Advent)