.... We have here two working principles. The first is this: no one is so blind to his own sins as the man who has the most sin upon him. If a man is plague-stricken, he can see it by the discoloration of the skin. If the scales of leprosy are coming up upon his arm, he can tell that he is a leper. If a cloud is growing over the pupil of the eyes, he can tell that he is losing the light of heaven. All the diseases of the body make themselves known emphatically; but it is the subtlety and danger and deadliness of sin that it conceals itself. No men know the light of God's presence so little as those who are covered with sin; and the more sin they have upon them the less they can see it. Though all the perfections of God, like the rays of the sun which encircle the head of the blind man, are round about them all the day long, they are unconscious of His presence. They are like Elymas, the magician, who, for his impiety, had scales upon his eyes; and because they do not see the light of God, therefore they do not see His perfections, and therefore they do not see themselves; for the light of the knowledge of self comes from the light of the knowledge of God. How shall a man know what unholiness is, if he does not know what holiness is? How shall he know what falsehood is, if he does not what truth is; or impurity, if he does not know purity; or impiety, if he does not know the duty we owe to God, and the majesty of God, to Whom worship is due? Just in the proportion in which the light of the perfections of God is clouded, we lose the light of the knowledge of ourselves; and the end of it is that when men hear... "That is just the character of my neighbor - that is the very picture of my brother"; they do not see themselves in the glass. You may describe their character, and they will not recognize it; you may tell them, "This is your self," and they will not believe it. There is something within them which darkens the conscience; and why is it? Because sin stupefies the intellect and the heart: it draws a veil and a mist over the brightness of the intelligence, and it darkens the light of the conscience. Sin is like hemlock: it deadens the sense, so that the spiritual eye begins to close, and the spiritual ear becomes heavy, and the heart grows drowsy. And when men have brought themselves to that state by their own free will, then comes the judgment of God: I will give them eyes that they may not see, ears that they may not hear, hearts that they may not understand, lest they should be converted, and I should heal them. These things said Isaias, when he saw His glory and spoke of Him (Jn. 12.40-41). [From a Sermon of Henry Edward Cardinal Manning]
Related post: "On Impenitence". See also: "Sin And Its Malice (I)", "On the Remorse of the Damned", "On the Number of Sins Beyond Which God Pardons No More"