My dad is going insane, but my family think's there is nothing we can do. Any advice?

Hi all, my Dad thinks that the world is ending (the biblical rapture) near the end of September, he even has it down to the minute. My sister in law, brother, and myself believe he is suffering from "religious obsessions / religious delusions", because all he does is watch conspiracy youtube videos about how the Bible is telling us when the rapture / resurrection will be, he fills notebooks with his notes, thoughts, and theories. His bible is full of his off the wall theories, written in the margins, etc. When we take him to family gatherings, this is all he talks about, and it makes people really uncomfortable. This is all he wants to talk about to my mother (they live together) and myself (I visit a few times a month). It causes my mom a lot of unneeded stress, and it causes fighting between my father and myself. My father has lost all of his friends BECAUSE this is all he talks about, and people tell him he sounds crazy, and he just smuggle says that he's right, and we'll all see (that the world is ending at the end of September).

My father has recently taken to spending all of their life savings, since he thinks the world is ending in a few weeks. This is a HUGE problem since my mom also helped to save up that money for retirement, and will need it, and it wasn't enough to support them to begin with!! I've been urging my mother to move all of the money to a bank account with only her name on it; she says I'll think about it, and then does nothing. My mother is in denial; he just ignores all the crazy stuff he says and keeps moving along with her life. She says there's nothing we can do.

I've spoken to my brother and he also says there's nothing we can do. I've talked to my father A LOT OF TIMES about going to see a mental health care professional "just to see if everything is right", etc., but he will not go no matter what I say.

I have booked a hotel room in the city they live in, so that I will be there the morning he thinks the world is ending, but I'm not sure what to do. My mother plans on going to work as usual, because she doesn't want to feed into his fantasies. I've been urging her to stay at the hotel with me, because I don't know how he will act. He's a complete fanatic! I'm not even sure I should be around him the morning he thinks the rapture is coming, because I'm not sure if it will be safe.

He also believes that police / some group of people are going to be searching everyones houses, and has mentioned that he will be on guard with anyone who knocks on his door after the rapture. He thinks that a lot of pillaging will essentially ensue after the "rapture", so he has mentioned taking his guns out and being "ready".

The police in their city are absolutely incompetent / power crazy. I'm afraid if I were to call them for help, they would end up aggressing my dad / causing something horrible to happen. I don't know who to call for help. If he were to get put into a mental institution, we couldn't afford it.

What should I do?!?