@HappyEmu religious psychosis is very common. When I was in hospital we had someone who used to walk down the street waving a cutlass converting people. He got lots of converts but wound up being apprehended by police and put in hospital! Its very distressing for both family and onlookers. When I was in hospital all they did was heavily medicate him. He ended up taking his life. He was a nice young man but he suffered greatly because of psychosis.
I think now that I am 56 and explored a lot of religious faiths all I can say is to provide tolerance and just allow him to be no matter how disagreeable his actions are to people. He's found faith, his beliefs may be extreme but you will only end up alienating him by providing direction. All you can do is just be an onlooker and not try to control an outcome.
He may have friends in his place of worship but it sounds like the religious institution to which he belongs is very prescriptive and cult-like in terms of who is admissable. Thats the way they operate. They cut off friendships and family because they are not religious practitioners themselves and indulge in practices that the church deems not part of their doctrine.
Dont try and change him dont criticise unless you want to drive your family further apart. Just be there and dont try and sway him. These places of worship will offer him lots of support though in terms of fellowship so he will have people to mingle with and offer companionship. His former friends may not have been genuine anyway and walked away due to his psychosis. I understand its hard for a parent to watch but you cant put an old head on young shoulders.
There is a classic book called Stages of Faith which is a psychological analysis of faith - its not religious based but talks about the development of faith in people and how it grows into a mature faith, one of universalising compassion and love. It takes years to arrive at that point. Its not a book about religious psychosis but provides insight into the maturity of faith by a psychologist. He is stuck in a developmental stage with his faith and all you can do is just allow him to grow and mature as disagreeable as his actions are to you. Given the right conditions he will take the correct steps and move into the direction of universalising love and compassion as he grows older. Just be patient i realise you dont like it but its just a part of who he is becoming as a person.
