When somebody says "I believe in God" it means that they believe in the existence of, or have faith in God. If I say "I believe you", it means that I believe what you have told me. If I say "I believe in you", however, it doesn't mean that I believe you exist but that I trust you or have faith in you.
:up: But 'believe in' has other meanings - particularly 'espouse/regard as important'. If someone says 'I don't believe in sex before marriage' it's not a sign of their naivety! (I'm told that sex before marriage has been known to occur.)