I'm an atheist. The joke calls me and all other atheists a fool. There's no basis to that accusation, hence it's insulting. It's no different to telling a joke that calls black men stupid or Arabs idiots. I doubt you'd post a joke with either of those as the punchline.

I'm not personally insulted by this because I have a grip on reality and am able to ignore such slights. My point is that it's not a joke and shouldn't be presented as one.