Note the attempt to ring fence his first claim from critical scrutiny, by making it vague to the point where it becomes unfalsifiable. Nothing new there of course such irrational arguments abound, and here comes the fallacy that makes it irrational:
So prima facie he sets up an unfalsifiable concept, and then creates an argumentum ad ignorantiam fallacy, by implying his claim ( a deity exists) is true because atheism fails to offer a rational argument disproving it.
Of course on top of this @christianapologist is using a false equivalence fallacy, since materialism has nothing whatsoever to do with atheism. And of course anyone with any grasp of logic will know why it is an argumentum ad ignorantiam fallacy, since atheism need explain nothing, as it is solely a lack or absence of belief in any deity or deities. One wonders what exactly his lack of belief in mermaids explains? One assumes nothing, does @christianapologist then rationally imply mermaids are real? If not then a special pleading fallacy is inevitable.