Is anybody a caregiver to a loved one with religious obsession?

If this is new for him, then more than likely his dementia has deepened this religiosity. What is disturbing about it is more than he is concentrating on the sort of "old testament" God-as-wrath, rather than the new testament teaching of resurrection and love and forgiveness.
Since your Dad is a Catholic I would go to his local Parish and ask your most kind Priest to come and visit with him. They would have a good conversation, I would think. You could also probably help by reading him the more beautiful passages from the bible, those that are uplifting, and of joy. Try to steer his belief toward the positive messages given.
Most of my nursing career was spent at a Catholic Hospital where we had a roving Nun on the floors at all time. She was a marvelous woman, salt of the earth and both feet on the ground. She was such a comfort to ALL patients, but most of all to those suffering from religiosity combined with mental illness. A marvelous soul she was, and full of gentle humor. I saw her presence calm many a troubled soul.

With your Dads age and his deafness, I would say there is some cognitive impairment. Its been proven that the deaf will likely have a Dementia.

I am not Catholic but I know when your Dad was growing up the religion was much stricter. My Aunt was married in 1958 and had to get permission to marry my Protestant Uncle and have her Protestant BF stand with her. My Uncle had to sign something saying my cousins would be raised Catholic.

Maybe Dad needs to hear that God forgives. God realizes we are not perfect and never will be. As long as we are aware of our shortcomings, we strive to be better, we ask God to forgive us and mean it, to me thats all God asks of me. We learn thru our mistakes and hopefully are better for them.

I am divorced. God gave me a good man the second time around. We've been married 40 yrs.

We are a Catholic family also. I think obsession can happen in any denomination or a non denominational church as well.

I know an older woman who is like your dad.

She constantly talks about Jesus, Mary, Joseph and all of the angels and saints!

I decided to ask her why she constantly spoke about religious topics.

I was curious what motivated her to speak endlessly about religion.

I listened without judging her. I was surprised to find out her reason for her obsession.

Her faith was a great comfort to her but she wasn’t at peace. She was desperately trying to settle things in her heart.

She was afraid to die. She was riddled with guilt about what she perceived as grievous ‘sins’ that she committed.

She had been divorced. I knew that she was abused in her marriage.

I explained that the church did not frown on people who left a marriage due to abuse.

I encouraged her to speak to a priest to learn how things stand now in the church.

A long time ago things were handled differently in all religions.

Divorce was frowned upon. Other things were too.

I think sometimes older people get stuck in mindsets from the past.

Sometimes people are fanatics. Was your dad always like this or is it a new behavior?

It is always a good idea to check if there are medical reasons for certain behaviors.

Best wishes to you and your dad.

Could be some cognitive decline combined with all the isolation of this past year.

My mom is a devout Catholic living in a Catholic Assited Living facility. The events of 2020 have convinced her that we are indeed now in the end times. The clergy there have encountered this often amongst the residents recently and attempt to provide comfort. It is a good suggestion above that if the priest is able to see your dad or at least call and provide some reassurance that may help.

It was difficult for all the residents at my mom's place as they could not attend daily mass due to being on lockdown. She finally figured out the in house video mass they did daily and watches a lot of EWTN.

The interruption of their previous daily routine and inability to see family has made their already small world, smaller and she focuses on negative things like that. It's hard not to be obsessed and draw conclusions. All the news is so negative. I've had to take a few news breaks myself.

The priest there suggested that she say an extra rosary or two and the reciting of it does seem to calm her down but I imagine if your dad is devout he probably is already doing that. The priest at her place suggested that we call her and say the rosary with her over the phone and despite hearing challenges she likes that. I am not such a good Catholic girl anymore but I have found it soothing also.

Blessings to you and your dad. I hope he finds some comfort and reassurance and that you do as well. Those are difficult conversations to have with a parent.

I think it would be a good time for you to call his parish priest and explain what's going on with your dad. I would even request that he come to the house and talk to your dad. There's no way you or your siblings will be able to change his belief but maybe someone with a better understanding of the interpretation of the bible might. Call his pastor.

It's not necessarily a dementia-related thing. Was your father a devout Catholic during his life? Maybe now that he's old he is probably thinking about it more and more. Ask a priest to come and see him and he will counsel him in the faith. It will help your father and put him a bit at ease. With Covid going on, I don't know if priests still can go to a person's house and administer to them. They probably would by video call or phone call though.