I agree with you that the ex-[insert religion here] content tends to be better informed on that specific group and brings a valuable perspective due to their experiences. Bryce Blankenagel is one example for Mormonism that comes to mind for me, I enjoy hearing what he has to say on their history and practices. At the same time, I don't think it's necessary to have been part of those groups or to know every facet of their belief system to post about them, particularly in a meme community such as this. It certainly helps when you know the subject material but growing up in a different community and environment does not disqualify you from being able to criticize it.

As it relates to this chain of replies, someone raised around mostly Christian influences who gets on here to poke fun at Islam or Judaism is less likely to be speaking from complete ignorance since they share common origins. You have to go further back to find it than when Catholics and Protestants are trading barbs but, as Pons_Aelius points out, they're Abrahamic religions and have no shortage of common features worthy of ridicule.