theism does make relatively understandable claims.

Depends what you mean by "understandable". It seems to make claims that most people think they understand, but I am inclined to think that this is an illusion, that actually religious language only has connotations and doesn't denote anything. As teageegeepea references below, religion is "not even wrong". This case is made pretty strongly at the start of George H Smith's "Atheism: the case against God".