11 Do we think we have knocked? Is there raised up within us something whereby we may even slightly conjecture whence light may come to us? It is my opinion, brethren, that when we speak of these things, and meditate upon them, we are exercising ourselves. And when we are exercising ourselves, and are as it were bent back again by our own weight to our customary thoughts, we are like weak-eyed persons, when they are brought forth to see the light, if perchance they had no sight at all before, and begin in some sort to recover their sight by the assiduous care of physicians.
And when the physician would test the progress of recovery, he tries to show them something which they sought to see, but could not while they were blind: and while the eyesight is now somewhat recovered, they are brought forth to the light; and as they see it, are beaten back in a manner by the very glare; and they answer the physician, as he points out the object, This moment I did see, but now I cannot. What then does the physician? He brings them back to their usual ways, and applies the eye-salve to nourish the longing for seeing that which was seen only for a moment, so that by the very longing he may cure more completely; and if any stinging salves are applied for the recovery of sound ness, let the patient bear it bravely, and, inflamed with love of the light, say to himself, When will it be that with strong eyes I shall see what with sore and weak eyes I could not?
He urges the physician, and begs him to heal him. Therefore, brethren, if, it may be, something like this has taken place in your hearts, if somehow you have raised your heart to see the Word, and, beaten back by its light, you have fallen back to your wonted ways; pray the Physician to apply sharp salves, the precepts of righteousness. There is that which you may see, but not that whereby you can see. You did not believe me before that there is that which you may see: you are now, as by the guidance of reason, brought to it: you have drawn near, strained your eyes to see it, throbbed, and shrunk back.
You know for certain that there is what you may see, but that you are not yet meet to see it. Therefore be healed. What are the eye-salves? Do not lie, do not swear falsely, do not commit adultery, do not steal, do not defraud. But you are used to these, and it is with some pain you are drawn away from old habits: this is what bites, but yet heals. For I tell you freely, by fear of myself and of you, if you give up the healing, and scorn to become meet to enjoy this light, by weakness of your eyes, you will love darkness; and by loving darkness, wilt remain in darkness; and by remaining in darkness, will be cast even into outer darkness: there shall be weeping and gnashing of teeth. If the love of light has effected nothing in you, let the fear of pain effect something.
Source: Tractates on the Gospel of John (New Advent)