I think one should always doubt their belief in God, because what is belief made from? Finite conceptions of an infinite truth are always wrong to some degree.
For example, say you had a picture of a beautiful painting, like Van Gogh’s Starry Night. Now focus in on a single atom within the painting, so that one atom is all you see, and all you’ve ever seen. What do you think about the painting then? Whatever you think, however true it might be for an individual atom, it’s not true for the painting as a whole.
Zoom out until you can see a few molecules: still the case; zoom out to a drop of the paint: still the case; zoom out until you can see one star: still the case. Etc.
As it happens, in this analogy, there’s a point at which we’ve zoomed out enough to see the whole painting, and then it makes sense. But what if the subject of our attention is infinite, and there’s no such thing as zooming out far enough? Then, no matter how true our thoughts might be about the small part we know, it is not the truth of the whole.
Therefore, doubt your beliefs about God every second of every day. They are like clothes: put them on in the morning, take them off at night, and change them the next day. As long as your mind keeps growing, your ideas should keep changing, as more and more of the picture comes into view.
Note: I do think this is a separate question from belief in a moral code, or the teachings of a Prophet, since those two are local to our experience, and relative to the world in which we live. I’m more talking about our beliefs in the Divine Entity behind it all, about Whom shall always know next to nothing.