Not sure if this is helpful or not, but I think there’s a qualitative difference. Nobody builds a whole belief system around fairies. They are stories or legends that don’t have a whole lot of depth to them. True religious belief is something much deeper, which often leads to, and benefits from, deep reflection and reasoned understanding. Nobody orients their life around fairies, unless they’re trying to get on “Wife Swap”.
I’ve heard this line of reasoning before… draw an analogy between God and clearly mythical or legendary things like fairies. Or make up something patently absurd (“flying spaghetti monster”) and invent some ridiculous beliefs and practices and say “see, I invented an absurd religion. Thus all religions are absurd.” Nowadays we don’t talk so much about fairies, that’s just old quaint nonsense, but those old-timey folks were pretty gullible and didn’t have modern science to explain how the world worked, nor TV to fill their imaginations with high quality dramas and lifelike characters, so of course they just made all kinds of stuff up and really believed it, whether it’s fairies, God, angels, it’s all the same.
I’m not a student of folklore, but things like fairies and what not - I’m not sure the extent to which people really ever believe in those things, or just like to talk about them and tell stories because that’s what people do, we like a good story to tell our kids with a wink-wink, like we still do today about Santa Claus, or Ninja Turtles. Those of us who teach our kids about both God and Santa Claus know that there’s a difference between the two, and the kids figure out the difference pretty quickly too, but they still like making believe about Santa Claus. Of course there is a grey area in between, what I call “superstition”, which as I see it are beliefs in legendary things that people never quite move beyond that four-year-old acceptance of. You see the adults who really believe in the Evil Eye and so forth. But I think that’s a bit of an anomaly… there is stuff that doesn’t stand up to critical analysis. Religions that have stood the test of time do. That of course doesn’t mean they’re true… I happen to believe Christianity is… but that’s another matter.