3 And now, because of certain points in the lesson before us, let us run over the words of the Lord, and of the whole lesson itself rather than make them a theme of discourse. “As He passed out, He saw a man who was blind;” blind, not from any cause whatever, but “from his birth.” “And His disciples asked Him, Rabbi.” You know that “Rabbi” is Master. They called Him Master, because they desired to learn. The question, at all events, they proposed to the Lord as a master, “Who did sin, this man, or his parents, that he was born blind?”
Jesus answered, “Neither has this man sinned, nor his parents,” that he was born blind. What is this that He has said? If no man is sinless, were the parents of this blind man without sin? Was he himself either born without original sin, or had he committed none in the course of his lifetime? Because his eyes were closed, had his lusts lost their wakefulness? How many evils are done by the blind? From what evil does an evil mind abstain, even though the eyes are closed? He could not see, but he knew how to think, and perchance to lust after something which his blindness hindered him from attaining, and so still in his heart to be judged by the searcher of hearts.
If, then, both his parents had sin, and the man himself had sin, wherefore said the Lord, “Neither has this man sinned, nor his parents,” but only in respect to the point on which he was questioned, “that he was born blind”? For his parents had sin; but not by reason of the sin itself did it come about that he was born blind. If, then, it was not through the parents' sin that he was born blind, why was he born blind? Listen to the Master as He teaches. He seeks one who believes, to give him understanding. He Himself tells us the reason why that man was born blind: “Neither has this man sinned,” He says, “nor his parents: but that the works of God should be made manifest in him.”
Source: Tractates on the Gospel of John (New Advent)