You don't have to have the same level of understanding for adding something in your story as you would if you were writing a research paper. Because, oftentimes, if people really wanted to know more, they can research it at their leisure and won't ask you anyway.
Spiritualism for example. I read what it was and its origins and its principles. It's literally just, when it was founded, by who and what people who follow it believe in.
Adding info to a novel isn't like creating a presentation or lesson plan. The information will always be there. And my readers don't quiz me on what's provided in text. And if that information isn't hard to follow, you can search the parts you don't understand.
Slang is a bit harder, but the definitions and context are given in every list I have saved. Most are things that haven't been lost to antiquity and have more than enough context.
And no matter what fictional thing you add, people will have something to say. Fiction isn't absolved from criticism. No matter what you include, it's compared to and inspired by something real.