The Argument from Efficient Causality
Everything that exists does so due to some cause. The only thing that might conceivably exist without a cause is a supernatural being which is self-existent. Therefore, Cthulhu must exist: an uncaused being who could move to cause the universe.
(I apologize/say "you're welcome" for trimming these arguments of their excess curlicues of language and making them slightly more coherent. I suspect part of the apologist's tactic is to give people a headache, making them give up and assume he must know better than they do.)