I didn't join either "church" at character creation as I wanted to explore both before choosing. However it looks like there is no way to join in game? I have a big interest in religions created in media and I might restart the game before I miss out. On xbox so mods won't save me. Internet doesn't seem to have a clear answer. Has anyone found a way to join or redo traits?
There is a guy in New Atlantis who gives you a quest and, if you finish it, invites you to his church. Is that Sanctum? I don't remember, my character wasn't interested in joining so I didn't try.
I have my own ideas about what church he is for since one of the books he wants you to read is about grav jumps…which if you remember one of the hostile religions believe they can communicate with their god during grav jumps