Science fiction is mostly set in future worlds, but we cannot know the future in advance. To create a credible future world, you have to be able to build on what has happened in the past to see what might happen in the future.
Science fiction authors tend to do this faithfully as regards things like future technology and weapons. But when it comes to religion, they tend to let their personal prejudices run wild. All the ‘good’ or notable people share the author’s ideas about religion, and the ‘bad’ people have religious ideas that the author doesn’t like. They don’t take the time to be serious futurists when it comes to the topic of faith.
In Elizabeth Moon’s Vatta’s War series, none of the religions we know today seem to exist— not even modern atheism. They have been replaced by a group of nontheistic religions that people seem to adhere to as if they were Christianity or Judaism or Hinduism. In other words, the people seem to attend their nontheistic churches and follow those churches’ codes of conduct. The one exception is the bad religion, the Miznarii. That seems to be theistic, at least in the mind of the one believer important enough to briefly serve as a viewpoint character. And they seem to generate acts of terrorism.
In the real world, people who have theistic beliefs go t their churches (or temples, mosques) and follow the religious codes that go with the religions because they believe God or the gods want them to do this and to live this way. People who have accepted the dogma that there is no God and therefore no God-imposed moral absolutes don’t tend to found ‘churches’ that preach about atheism and attend ‘church’ once a week. Why should they? Atheism is a minimalist faith and can generate no pressure to create religious institutions or impose atheist absolutes. Organizing atheists isn’t like herding cats, it’s like herding microscopic organisms that cannot perceive you.
A futurist who wants to create one or more nontheistic religions that function like the theistic faiths we have today needs to do some development, and explain how that could happen when it hasn’t happened yet for the plain reason that nontheistic religions just don’t generate an imperative for that sort of thing. We have to figure out a reason why these particular nontheistic people felt the need for an organized nontheism and a shared philosophy and/or moral code. Perhaps it might be an outgrowth of New Age type meditation or positive-attitude classes or seminars. Perhaps some dictatorial force wanted his people sorted out into religions with weekly religious service attendance to keep them in line.
Using history is a great tool for the futurist, but you must be aware that our culture has little sense of history and loves to change the history to fit current ideology without worrying about documenting the new version of history. Currently, Jews and Muslims don’t get along and Christians tend to really like Jews. But there was a time that Christians disdained Jews as Christ-rejecters and Muslim rulers were welcoming to their Jewish subjects. Which history will you base your future versions of these faiths upon? If you just project today’s attitudes into the far future, your fiction may start to look dated pretty quick when things change.
One important rule for the futurist: don’t make all your changes to history ones that you like. If you are a well-informed, university-educated Christian who dissents from evolutionary theory, and you create fictional worlds where the scientific establishment has fully accepted your ideas, you are creating a fantasy future where things go the way YOU want. How about a future where one has to accept evolution and reject God in order to continue in school beyond third grade? Plenty of conflict for the Christian author to work with there. Or one in which Darwinism is rejected because some scientifically advanced alien race condemns it as ‘magical thinking’ and has their own, better theory— that they believe humans are too stupid to understand.
Secularists do a similar thing when they create futures in which all the religions that annoy them are extinct, or practiced only by the uneducated or by the bad guys. In the real world we often have to work together with people who have different ideas than we do, and fight against bad guys who are secularists-like-us or Christians-like-us.
So if you want realism in the religions of your future world, make sure there are both ‘good’ changes and ‘bad’ ones. Most of us would agree that slavery is bad and genocide is bad. What about if some religion in your world— perhaps your main character’s religion— accepts slavery or even genocide as OK in certain circumstances? What if the only religion speaking out against the evil is one that you, the author, have little use for?
One worldbuilding trick is to project a current day religious group into a future situation and see how or if they would change. For example, the Amish. In a spacefaring culture bent on colonizing new worlds, would the Amish have to change, adopting more technology into their lives? Or would they be so valued for their ability to do low-tech farming that they would be pressured NOT to change? (In my current WIP, I have some Amish sisters who lose their family farm in an attack, and then take up farming in a Fleet starship.)
Or how about the atheists? I mean real, thoughtful people who happen to believe that the evidence is against God existing, not those rude children who troll around on the internet. How would they cope with an alien race that insisted that it was easy to prove the existence of a God— or an atheist alien race that insisted that Terran atheists must kill all who disagree with them to achieve a ‘pure’ world?
A final factor to consider: how important is the religion factor to your worldbuilding? In adventure-based science fiction, religion may just be a bit of ‘set-decoration’ in the background of your space-war or quest story. In a more philosophical work, it may need more development. And if the religious/spiritual side of life— or the war against religion— is center stage, you will have a lot of work ahead of you.
Have you done your writing today? Maybe you should get started. Right now.