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 seen through a dirty telescope. That is why horrible nations have horrible religions: they have been looking at God through a dirty lens.
God can show Himself as He really is only to real men. And that means not simply to men who are individually good, but to men who are united together in a body, loving one another, helping one another, showing Him to one another. For that is what God meant humanity to be like; like players in one band, or organs in one body.
Consequently, the one really adequate instrument for learning about God is the whole Christian community, waiting for Him together. Christian brotherhood is, so to speak, the technical equipment for this science—the laboratory outfit. That is why all these people who turn up every few years with some patent simplified religion of their own as a substitute for the Christian tradition are really wasting time. Like a man who has no instrument but an old pair of field glasses setting out to put all the real astronomers right. He may be a clever chap—he may be cleverer than some of the real astronomers, but he is not giving himself a chance. And two years later everyone has forgotten all about him, but the real science is still going on.
Quotes from Mere Christianity, Part 65
C.S. Lewis, Mere Christianity (1952; Harper Collins: 2001) 164-165.