What discounts the many discounts the one?

At lunch today with four of my old friends (an atheist, an agnostic, a monotheist, and a polytheistic neopagan—see the Wikipedia article on the Ásatrú movement), I brought up the subject of religion by asking the atheist, "So, have you gotten religion?"

"No, I haven't," he said, possibly a little taken aback.

"I didn't think you had." I reminded him that he had told me some years ago that he was an atheist, but he didn't seem to remember it. I told him that his admission had made an impression on me and had been, along with Sam Harris's The End of Faith, a source of support in my own decision.

The neopaganist, who (like me) majored in philosophy, cited Bertrand Russell for having said something along the lines:

I approve the monotheist's reasons that there aren't many gods, but the reasons equally weigh against there being even one god.¹

Later, this friend said he couldn't run down the actual quote, which may be apocryphal. "A philosophy professor of mine long ago once referred to it offhandedly. As nearly as I can remember, here is the gist of the argument: Monotheism is the most nearly correct religious system because, mathematically, the number of gods it worships (i.e., one) is closest to the actual number of gods in existence (i.e., zero). Or words to that effect."

The monotheist, who manages to be so without being particularly hospitable to religion (as my Jehovah's Witness friend Tom Sheepandgoats says he isn't either), shared what appears to be another version of Russell's quip:

If you understand why you discount all the other gods but yours, then you understand why I discount yours as well.

He said he found this on some newsgroup or other at least ten years ago, credited to someone named Ken Hall. He said he himself likes to use a modified version of this on particularly staunch churchgoers he happens to meet:

If you understand why you don't accept any other church's dogma, then you will understand why I don't accept your church's dogma either.

The agnostic was quiet during lunch, but afterwards she sent me seven or eight links to related articles. But those are for another day....
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1. The atheist commented that he'd heard on talk radio years ago the quip that

   Unitarians believe there exists at most one god.

   He said it's attributed to Alfred North Whitehead (Bertrand Russell's co-author of Principia Mathematica).