If you want a fun monotheistic argument invented by atheists, try this one, which was published by Robert Meyer and attributed to Hilary Putnam. It's a clever use of the Axiom of Choice and Zorn's Lemma.
Isn't that just the First Cause argument, wrapped up in set-theory language?
Well yes, but it "addresses" one of the really basic responses to the First Cause argument, that there might - for all we know - be an infinite chain of causes of causes, extending infinitely far into the past. One of the premises of Meyer's argument is that any such chain itself has a cause (i.e. something supporting the whole chain). That cause might in turn have a cause and so on. However, by an application of Zorn's Lemma you can show that there must be an uncaused cause somewhere in the system.
If you don't assume the Axiom of Choice you don...
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