What I don't believe exists is anything non-physical. Simply because I have no reason to believe in anything non-physical.
How I define physical is anything which can be detected by our senses or can affect something which we can detect by our senses.
Therefore anything claimed as supernatural or divine is imaginary to me. I understand other people believe in a reality which includes spiritual/non-physical elements. However in an argument or discussion these non-physical concepts have no significant meaning or explanatory value.
I don't mean this offensively, one has to choose for themselves what they are willing to accept. However this is how my mind works in discussions.
I used to be a Development Chemical Engineer. My job was to come up with new ways to solve environmental problems, but at the EPA compliance levels, that were 10 years in the future. This meant I had often had to start with little in the way of what already existed in material reality; not yet seen There was often nothing off the shelf, since that tech would not be compliant in 10 years and may not extrapolate. Rather I had to reverse engineer old problems to new solutions, that could achieve future standards. This was to help the nuclear industries in terms of environmental compliance; future is now.
This blue sky research; no pressure, only works, if you have faith in what is not yet seen. I did not initially know what had to be done, but only that I felt I could do it, somehow. Like the artist, the process often begin as a hunch, and then maybe a few baby steps and a few falls, toward the goal until I am walking toward the light. Without faith in that which was yet to be seen, there would be no Pyramids, as well as no modern creature comforts and breakthroughs in science and technology.
It is not until these final objects appear, fully assembled in material reality, that some start to believe. However, they miss a lot of the front end excitement connected to discovery, exploring outside the visible box. Your box so too small for me. I would feel claustrophobic. My mind wishes to see what lies in the future, that is not yet visible to those in the box. I found my niche.