"I don't know if a god exists, but I believe one does"
Isn't that the whole idea of "religious faith", though?
It certainly is not a position you'll find among fundamentalists. Those people will not only all claim to know, they'll even claim certainty.
But it seems to me that the more "sophisticated"/"educated"/"reasonable" theists almost always will say that they don't claim to know. I certainly distinctly remember Ken Miller, for example, say exactly that. These people understand that "personal evidence" isn't usefull evidence to convince other people and that "knowledge" is something that you can share and demonstrate.
Danneels, a retired catholic cardinal in Belgium, literally said once "it's not knowledge like knowledge of gravity, it's faith".