Even if you change that to core flaw in most religion, I am not sure I would agree. It doesn't really matter whether the rank and file study this or that. I think it's more a matter of their attitudes towards others, not how much they actually know. And that varies a lot.
The idea that there is one exclusive right religion and the others are all nonsense is Jewish invention from late Old Testament times. Even then they didn't care what other nations believed one way or the other, and Jews today mostly still don't.. Christianity then Islam picked up the idea and added that everybody had to follow the one true religion. In the East and pre-Christian West there was no idea of exclusivity and a much more free exchange of ideas. Modern liberal Christians aren't exclusive and many don't believe other religions are nonsense, they just say they are different paths.
So I stand by my statement that.exclusivity is not a core flaw of religion, just some versions of Western religion. Specifically, conservative versions.
Which leads me to what I do think is a core flaw of religion. It tends to be conservative and hierarchical. It's got a built-in conservativism because it is looking to a past when an absolute truth was revealed. As I said, you do get liberal religion, but liberals have to overcome the conservative bias to get there.
= new reply since forum marked as read