The book probably has nothing to do with Christian Catholicism, and is not a religious sect but a book. That’s a common mistake people make. Catholic is a word outside of that particular sect of Christianity. It means “universal.” In the context of Dune, the Orange Catholic Bible refers to a universal Bible that covers many religions.

It was produced after the Butlerian Jihad to be a basis for everyone to use. There are not “Orange Catholics” as far as I’m aware. Orange and catholic are just descriptions of this essentially encyclopedia of religious texts for people to form their new understanding with after learning to think for themselves once again.