The moon... a ball of dirt that reflects the light of the sun into this world. I have heard the analogy that the Church's purpose is much the same - as men, we are only dirt that God has exalted to reflect the light of the Son (and His Gospel) into this dark world.
However, I was thinking recently about this analogy, and the thought came to mind that the moon does not always shine fully; we see only a half moon at times, a fingernail sliver at times, and there are those times that the moon is completely eclipsed.
Well if you're at all familiar with why we see the moon differ in these ways; it is because the earth is between the sun, and the shadow of the earth covers the moon, resulting in only a portion of the moon reflecting the sun (with an eclipse, the shadow of the earth will cover the entire moon).
And so it is with us as Christians. The more we allow the world between us and God, the less we reflect the light of the Son and His Gospel.