"The light shines in the darkness, and the darkness has not overcome it" John 1:5
But, it may be, the dull hearts of some cannot yet receive this light. Their sins weigh them down, and they cannot discern it. Let them not think, however, that, because they cannot discern it, therefore it is not present with them. For they themselves, because of their sins, are darkness. Just as if you place a blind person in the sunshine, although the sun is present to him, yet he is absent from the sun; in the same way, every foolish man, every unrighteous man, every ungodly man, is blind in heart. [...] What course then ought such a one to take? Let him cleanse the eyes of his heart, that he may be able to see God. He will see Wisdom, for God is Wisdom itself, and it is written: 'Blessed are the clean of heart, for they shall see God (St. Augustine In Ioann. Evang., 1, 19)