Do you not yet understand or comprehend? Are your hearts hardened? (Mark 8)

Faith, the theologians say, is a certain and obscure habit of soul. It is an obscure habit because it brings us to believe divinely revealed truths that transcend every natural light and infinitely exceed all human understanding. As a result the excessive light of faith bestowed on a soul is darkness for it; a brighter light will eclipse and suppress a dimmer one. The sun so obscures all other lights that they do not seem to be lights at all when it is shining, and instead of affording vision to the eyes, it overwhelms, blinds, and deprives them of vision since its light is excessive and unproportioned to the visual faculty. Similarly, the light of faith in its abundance suppresses and overwhelms that of the intellect...

Another clearer example...: If those born blind were told about the nature of the colors white or yellow, they would understand absolutely nothing no matter how much instruction they received since they never saw these colors... Only the names of these colors would be grasped since the names are perceptible through hearing... Such is faith to the soul; it informs us of matters we have never seen or known... The light of natural knowledge does not show them to us... Yet we come to know it through hearing, by believing what faith teaches in blinding our natural light and bringing it in to submission. St. Paul states: "Faith comes through hearing" (Rm 10:17). This amounts to saying that faith is not a knowledge derived from the senses but an assent of the soul to what enters through hearing... Faith, manifestly, is a dark night for souls, but in this way it gives them light. The more darkness it brings on them, the more light it sheds. For by blinding it illumines them, according to those words of Isaiah: "If you do not believe, you will not understand" (Is 7:9). (St John of the Cross, Ascend to Mt Carmel, 2:3)