> I'm envious of those true believer kind of people.

> My father is one of them and he's held absurd ideas as 100% facts and we've had many nasty quarrels about it,

I am not even able to fathom how this is possible; unless someone is trying to convince you to join them in their belief, how on earth does a quarrel arise from differing beliefs?

I'm a lifelong atheist surrounded by religious family (and friends, too, TBH), and the only problem is when they refuse to take subtle hints that I am not interested in reading their book and I have to be blunt with them. And even then, that is not sufficient to start a quarrel!

Often we think someone is 100% sure but they only appear that way to us. Trying to change someone's thoughts by arguing with them never works.

Nasty quarrels might indicate an amount of uncertainty, or an amount of inability to articulate a thought. We often have ideas we don't really know why we have them, so we can help others to try to explain things to us in a way that helps them understand why too.

A "nasty quarrel" requires more than one side, and this other side is also responsible for the quarrel. I think its wise when trying to talk about difficult things to first identify and agree upon the small things you can both agree upon. If a conversation becomes heated it's no longer a conversation and you should get out before it gets worse. If you feel it's leading into fire and can still be salvaged you can then go back to these shared things and start again.

However a real conversation about ideas will also challenge and change your own view of the world. You might find your own ideas changing. People generally find this a psychologically painful process and will subconsciously resist such a movement. Generally we prefer to label the other as different, alien, us vs them. Having a quarrel is therefore even more likely as it means that your own psyche is protected from encounter with the dangerous other. Understanding that this also applies to the person you are talking with can also help reduce tensions and increase empathy. Again, starting from common shared baseline will help.

I think you are very close to explanation. Ideas in human minds can be presented as facts. If you decide that you are happy by some setting - that becomes a fact to you, while in reality that is a belief. The same about depression and sadness - you can get impacted by information you did not knew and would not be impacted if you were in blissful ignorance and some people choose exactly that choice. Some people get psychosis and their mind is hallucinating that they are on fire - that is real to them as what are your experiences, though those also are not based on facts, but serve as an information delivery to your brain.

The whole issue with human minds is that it is not built to deal with scientific facts, but with socium of other people. You can't use facts when operating with society - you have to use symbols, that they will associate with. And I think that the issue is with you(as it is my experience as well) - I can guarantee, that there are people, that will explain to your family members EXACTLY the same ideas, that you are trying to explain to them... and they will agree to that person - and not to you, because you are clearly doing it wrong.

You should try and and do what the OP is suggesting, i.e. to try and put yourself in your dad's shows and try to see the world the way he sees and understands it. I.e. this type of conversation goes both ways.