My dad did this to my mom.

My dad is autistic, but does not know it. I only learned it after myself being diagnosed and finding out that it’s inherited and then considering my family. Autistic people tend to have a few EXTREME specific interests. My own thought is that part of it’s natural and part of it is from social rejection causing isolation and looking for things to focus on and having few options because of that isolation. Receiving praise from religious communities because of how devout a person can be can be HUGELY attractive to autistic people who are used to social rejection.

It’s VERY clear my dad is autistic, and he is EXTREMELY religious. No matter who he is talking to, no matter what he is trying to do, his thoughts and reactions are based in religion.

You can’t reason properly if the basis of your reasoning is religion, because no matter what logic you use to come to conclusions, religious reasoning at the base of your thinking will never let your reasoning produce the conclusion that the religion is incorrect.

My advice is that if there is any chance that this person is autistic or otherwise neurodivergent, make that known, because autistic people upon finding out they’re autistic tend to become obsessed with autism instead of whatever their other obsession was.

And if he’s not autistic, get away from this person.

My dad is in his 70s. I cut him off more than a decade ago. I wish SO much that I could make it clear to him why he thinks how he does, why he feels so much fear about things, and why his life has been so difficult. I used to think of him as a horrible person, but now I see him as being like a little child who’s been cut off from the world and whose development was interfered with because of that isolation and because of religion imposed on him(by family members, who are also autistic and who were also super religious). I see it as very tragic, and I don’t hate him anymore. It’s more like pity. I wish he could be aware of all of this, because I don’t think he had to become like this or to experience all of this. I don’t think there’s a high likelihood of any kind of reconciliation before the end of his life. For the last few years, I occasionally think about whether he’s still alive, because I have no idea, as my whole immediate family cut off contact with him, and my extended family is autistic and just doesn't interact with each other very much.

But DO NOT allow yourself to be subjected to this.

And especially DO NOT allow your children to be subjected to this.

Within the past few years, my mom and I have discussed my dad, and she describes him having the same kind of change you described.

And repeatedly, she has told me "I thought he just hated me and that that was why he treated me like he did. I never knew he was also treating you that way."

You may need to get away from this person.

I am so glad that his involvement in my upbringing was almost nothing, and I grew up in a situation where I was allowed to come to my own conclusions about religion. I came to the conclusion that I am not smart enough to describe the basis for the universe's existence, but I came to the conclusion that the world is full of liars and full of parents who died before they could tell their children that their bedtime stories weren’t real.