There is an excellent little speech by Douglas Adams here:
http://www.biota.org/people/douglasadams/
(Scroll down to where it says "Where does the idea of God come from?") Ultimately, though, you should decide for yourself whether you believe in a certain thing or not (e.g. Loch Ness monster, crop circles, God). The funniest thing is, whichever "side" you're on, it will always be blindingly obvious that you are correct and the other side is wrong.
If there is "God", as you clearly state, then of course it exists.
As to whether there is such a thing as "God" well that's a completely different question.