There are many bad folk arguments for the existence of God that depend on bare assertion, but the worst of the classical arguments IMO is the argument from design. It seems obvious to me that the universe just isn't that intelligently designed as one would expect from a Creator. Plus we can explain too many of its features naturalistically now.
I mean, these "naturalistic" explanations aren't really the end of the story though, are they? Even if you take them at face value, they ultimately boil down to certain properties of the universe being set just right for all of these natural processes to occur. Tweak even one of the fundamental constants by a trivial amount, and suddenly the universe becomes a lot more... boring, so to speak. Now, plenty of scientists have criticized the idea that the fundamental constants are "fine-tuned" (or even that such a claim has any meaning given that we can't observe any other universe). Personally, I think the anthropic principle is more than sufficient to explain any otherwise freakish coincidence that allows for the existence of life in our universe on a theoretical level. But again, any argument for or against the existence of God is emotional in nature, and all these rebuttals to the fine-turning argument are emotionally uncompelling to someone who is seriously swayed by it. So in that light, I think being swayed into faith by it makes plenty of sense.
That's just the anthropic principle though.
