agnostic
An agnostic believes that it is not possible to know anything about God, let alone whether She exists or not. (The word means, more or less literally, "not-knowing.") An agnostic is not the same as an atheist, who would insist that there is no deity at all. Moreover, agnostics have been some of the world's great mystics, claiming that while the human intelligence cannot really know anything about God, divinity might be apprehended by other means. Rather than implying doubt, then, agnosticism could be considered a particularly strenous form of piety.
In common parlance, of course, agnosticism simply refers to the refusal to declare an opinion...