A post over at Evn's place got commenters thinking about fictional religions. My favourite fictional SF religion is the earth-based one depicted in Always Coming Home by Ursula Le Guin. My second and third favourites also appear in her books (The Telling and short stories about the people of Ki'o). I also rather like the religion of Minbar in Babylon 5.
And then there was a post over at Bo's place about artificial languages, specifically Brithenig, which reminded me that there is even a Language Construction Kit.
It's probably fun to construct both fictional languages and fictional religions (and there is a similarity between language and religion) because it helps us to think about how they are structured, what makes a good or bad language or religion, and whether other species would have anything that we could recognise as religion, and how they would communicate (maybe using pheromones, like the aliens in Liz Williams' Empire of Bones).