Two commenters with delightful names, Mark Christianson and PSanafterthought, have both nailed us dead to rights. Here, to summarize a bit, is their objection: Our different thoughts about "God" don't affect the reality of God in the slightest.
They are absolutely right. Vocatus atque non vocatus, Deus aderit, as the PBS t-shirts say. To be fair, we thought about this while writing, and -- perhaps foolishly -- decided that trying to say it all would lead to an overlong post. The result, unfortunately, is one which makes Father Anonymous look like a bit of an atheist. It all sounds as though God were nothing more than what we say about God, when in fact the reverse is more likely to be true.
So let's try to clarify a bit.
We can take for granted that all religions attempt to model the transcendent reality, and claim that their model is the most accurate one. (Pardon the silly circumlocution for "worship God," in which we indulge briefly only as a reminder that some religions don't use "God" as part of their theological jargon.) It will be a while before we know who was right and who was wrong.
From this fact arises the cant expression that "we all worship the same God," a statement which is true only with regard to intent. We mean to worship the same God, in the sense that we mean to acknowledge the actual facts of the universe -- the ground of being, if you will. The problem is that the models are so different as to be irreconcilable.
To use Mark's appealing but misleading metaphor, if you and I both discover that we know a fellow named Sam, but I say he is an old man with an eyepatch and a cane, while you say he is a spectral reflection of Medusa's left nostril (whatever that even means), then it is safe to say that we don't actually know the same person. Or that if we do, we still have nothing about which to speak. It is not even likely that we know different aspects of the same person; and, frankly, I don't think you've ever met Sam, you big faker.
Prothero's point is that rooting a general theory of religion in a doctrine of God has proven to be an academic dead-end, with which the popular imagination has not yet caught up. Our own point, which may be an exaggeration for effect but by which we will nonetheless stand, is that even the seemingly small differences in doctrine are profoundly important.
To be sure, as we have already said and as Mark is determined to say more forcefully, there are families, and family resemblances. One can see at a glance that Lutherans and Presbyterians share an enormous amount, at least when compared to, say, animists and Zoroastrians. The mistake, in our opinion, is to minimize the significance of the differences. To do so leads not just to an over-easy ecumenism, but (much worse) to the reduction of "God" from a semantic model for transcendent reality to a slogan which can be used and abused in the public realm.
It may be helpful to explain that our thinking here is shaped, at least a little bit, by Karl Barth's famous aphorism that "all religion is unbelief." (Having attended a school where Barth was frequently mistaken for God, we generally avoid the guy, but in this one case he is inevitable). Here's a quick synopsis of the case, by George Wolfgang Forrell:
In [KD I:2, "The Revelation of God as the Abolition of Religion"], Barth refers to religion as unbelief and writes: "Revelation does not link up with a human religion which is already present and practiced. It contradicts it."
By the way, it is in the light of this point of view that we must understand the sympathy for atheism as practiced by Marxist communism on the part of Barth and his followers. To Barth the denial of "religion in general" or the God of the philosophers was not only excusable but actually praiseworthy. This explains among other things ... [Barth's] relative tolerance of Stalinist Communism. The Communists were atheists, i.e. they objected to "religion." ....
Readers of Barth, and at least arguably the man himself, have sometimes treated this insight as an attack on non-Christian religions. But all that fails to follow the genius of the argument, which is that God is, in essence, hidden; the various revelations claimed by the religious are equally incapable of proof. In that sense, although Barth would have raised an imperious finger here, all the different forms of Christianity are "unbelief," insofar as they fall short of the truth. (And insofar as they disagree, they must fall short of the truth.) Meanwhile, beyond the documents and traditions which contain the "official" versions of the world's religions, there is the image of God treasured in the heart of the individual believer, which is beyond doubt the idiosyncratic product of communal tradition, individual psychology, and so forth. It is also the most elementary form of the "unbelief" Barth is talking about.
Does that help? "Religion in general" is the mistake. And there's a lot of that going around, as Christian ministers are reminded every time some public official invites them to offer a prayer at the town council meeting, and then mutters, "But try not not to mention Jesus."