Conan Doyle was also an ardent believer in Spiritualism (talking to the dead through seances). Actually that wasn't too unusual during the post WW1 period where a lot of people had lost sons in the War, and found it comforting to go to a seance and get 'messages' from Johnny.
One of Conan Doyle's books in his 'Professor Challenger' series mentions the Russellites, so he knew about them.