I'm not sure how strong an argument it is to point out, "If religion X is true, and God has been 'clearly revealed' by Y, a central belief within X, then why are there Z number of denominations?"
That is, in particular, if there is such a thing as "The God of the Bible," then why are there so many differing interpretations of the Bible among varying religions, and so many differing interpretations of the Bible within the same religions (denominations)? So you have different religions all using the same Book in very different ways, and then members of the same religion believing very different things about the same Book.
Although the number of Christian denominations is not nearly as high as sometimes gets cited, (probably around 9,000 Protestant, 200 Catholic, 800 Orthodox) and although some differences between the denominations are fairly trivial, there are certainly some large differences between some denominations. One may believe, I suppose, that their god wants a degree of liberty and individual pursuit of truth (which will lead to differences) on non-essential parts of religion X. In that case, though, it is difficult for me to believe X could be an "organized" or institutional religion.
On the other hand, if humans wrote the Bible with nothing divine at all to do with it, then we'd expect some vagueness and ability to interpret "the truth" in a number of different ways, which is what seems to be the case.
