There is a lot I love about Dune, both Frank Herbert's original novels and the later work by his son Brian Herbert and Kevin J. Anderson. At the moment I'm reading Sisterhood of Dune which is set many thousands of years before Dune itself and there's a section dealing with (and finally explaining, at least to me who hasn't read all these books yet) the Orange Catholic Bible.
You see, from bits and pieces here and there I understood that the Orange Catholic Bible was not the Bible and it turns out it isn't even a corrupted or altered future version of it, not really. Instead, its an attempt by the Imperium to impose a universal religion by merging all religions into a single faith. It went down poorly, by the way.
Its axiomatic that the world of Dune is all about doing Roman-style court politics in a sci-fi setting so having a sci-fi version of the Council of Nicaea in its past is a fantastic little worldbuilding detail.
There's also the idea of the Budislamic faith of the Fremen. Actually, there's the fact that not only are the Fremen Budislamic but they're specifically Zensunni. Not only have Buddhism and Islam merged at this point in history but there are subdivisions within the faith. That makes sense, after all the differing interpretations of, well, every religion going in the real world. Its actually something of a bugbear of mine that sci-fi and fantasy religions tend to be extremely and unthinkingly monodominant.
Its an incredibly minor detail but it lends an authenticity to ho a pretty fundamental aspect of human society works in that universe.