I dated a girl who's mom was very religious. I'm fine with that but was amazed how it controlled her life. To the point of who they could associate with. Later, a friend of mine married a lady who was also part of that same organization. Similar story. If you want to control someone, a good first step is isolate them.
(and there's my one and only religious post. last time I was banned it was in a technical thread for abuse of trolls. )
Indeed, religion serves as a collective narcissism, creating abusive, controlling, gaslighting relationships, designed to keep one on the inside, confused and passive. Perhaps even more insidious: while using the same psychological exploits as an abusive partner, it's largely self-imposed, by the theology itself. A reader might argue, this is pessimistic, or hyperbolic: "maybe that describes the most fanatical cults, but those are few, and mainstream religion doesn't fit that". Perhaps, but it's also impossible to tell from the inside. On the inside, it's "us vs. them", all against the forces of evil.
I've been fairly atheist for a long time, but this realization utterly sealed the deal for me. In fact this is one "personal change" moment I have a citation for -- TheraminTrees on YT is a psychologist with personal and professional experience with such relationships [i.e. personal and religious], regularly putting out videos discussing aspects of this dynamic. Unfortunately, there's not much you can do to help a person stuck in such a relationship -- it's up to them to make the escape. Be compassionate, feed them information when you can; be there for them if/when they decide to break out -- but also, don't throw yourself into it, and indeed, if they're very vocal about their thing, and highly resistant to change -- don't waste your time, preserve your sanity.
Tim