When someone tells me that they don't believe in God, I usually say, "That's interesting. Tell me about the kind of God you don't believe in." When they get over their surprise, they quickly warm to their task and tell me about the God to whom they were introduced as children...the guy in the sky; possibly angry, sometimes spying, always meddling. Sometimes they worked out for themselves at a very young age that sitting in the clouds was an impossible feet. Sometimes they came early to the conclusion that no God who could be called, "good" would cause them such pain and anguish. Sometimes they believed it all for a good long time, and then met science teachers or loving non-believers and then felt that they had been duped.
There are many good reasons for not believing in God. But there is no good reason for a grown up to insist that the guy in the sky is the only kind of God and that all believers in God believe in that kind. That's a patent falsehood about theology and about people, and UU's, dedicated as we are to truth, have no business espousing it.
For a quick and lively run-down of other kinds of God you might or might not believe in, check out this article, Got God? from the Winter UU World.