When you come to knowing God, the initiative lies on His side. If He does not show Himself, nothing you can do will enable you to find Him. And, in fact, He shows much more of Himself to some people than to others – not because He has favourites, but because it is impossible for Him to show Himself to a man whose whole mind and character are in the wrong condition. Just as sunlight, though it has no favourites, cannot be reflected in a dusty mirror as clearly as in a clean one.
You can put this another way by saying that while in other sciences the instruments you use are things external to yourself (things like microscopes and telescopes), the instrument through which you see God is your whole self. And if a man's self is not kept clean and bright, his glimpse of God will be blurred - like the Moon through a dirty telescope.
C.S. Lewis, Mere Christianity, p. 144