1 We have just read the long lesson of the man born blind, whom the Lord Jesus restored to the light; but were we to attempt handling the whole of it, and considering, according to our ability, each passage in a way proportionate to its worth, the day would be insufficient. Wherefore I ask and warn your Charity not to require any words of ours on those passages whose meaning is manifest; for it would be too protracted to linger at each. I proceed, therefore, to set forth briefly the mystery of this blind man's enlightenment.
All, certainly, that was done by our Lord Jesus Christ, both works and words, are worthy of our astonishment and admiration: His works, because they are facts; His words, because they are signs. If we reflect, then, on what is signified by the deed here done, that blind man is the human race; for this blindness had place in the first man, through sin, from whom we all draw our origin, not only in respect of death, but also of unrighteousness. For if unbelief is blindness, and faith enlightenment, whom did Christ find a believer at His coming?
Seeing that the apostle, belonging himself to the family of the prophets, says: “And we also in times past were by nature the children of wrath, even as others.” If “children of wrath,” then children of vengeance, children of punishment, children of hell. For how is it “by nature,” save that through the first man sinning moral evil rooted itself in us as a nature? If evil has so taken root within us, every man is born mentally blind. For if he sees, he has no need of a guide. If he does need one to guide and enlighten him, then is he blind from his birth.
Source: Tractates on the Gospel of John (New Advent)