I think it depends which God you ask if someone believe in. For example, the Platonic idea of The Good is a pretty abstract philosophical concept which does no

I think it depends which God you ask if someone believe in. For example, the Platonic idea of The Good is a pretty abstract philosophical concept which does not exist in the physical world - rather the physical world is some kind of shadow or reflection of this Good. You can't really prove or disprove the "existence" of this interpretation of reality, although some would say you can apply occams razor to it. Plato would argue that you cannot not belive in this, if you just consider the question reationally. On the other hand, the biblical God Yahweh is a beeing with a physical shape and with intentions and emotions, which walk around in the physical world in historical times and interact with people (and killing them left and right!). The existence of this beeing could easily be proved if you meet him, but cannot strictly speaking be disproved as long as he remains in hiding. But the evidence can be weighed the same way we weigh evidence for the existence of mammuths or unicorns. The christian notion of God has historically been kind of a mixture of these two ideas (and to a lesser degree many other). Due to this complexity of the concept "God", it is not even clear what it means to believe or don't believe in God.