I say that one cannot truthfully claim to believe in God. Why? Because by the nature of what God must be is an inscrutable, unfathomable, incomprehensible "thing" which has no human-known equivalent and different in order and kind from anything known in the material universe.
As such one cannot understand at all what they mean when they say "God." It's a meaningless (or its meaning is incomprehensible, which amounts to the same thing) term and holds as much significance as saying that one believes in "Splang." What the hell is Splang?
How can one believe in something they cannot comprehend? That sounds pretty impossible to me.
If one claims that God _is_ comprehensible in some way then their conception of God must be inaccurate or naive in some way and one could say that the individual believes in something which is not God and must be committing idolatry.
So one who says they believe in God must either be lying (possibly unintentionally) or an idolater.