Is there a demonstrable reason why one believes and the other doesn't? [Details inside].

I am not interested (during this question) in a religious debate/religion bash.
I have always deeply wondered what makes it that one person is susceptible to be a ‘believer’ ( in whatever religion) and the other is not.
I am aware that the word susceptible can have a judgemental sound to it, but it is the only one that I have.
Maybe I use it because I am looking for a fysiological reason?
Is there maybe a difference in fysiology/something in the brain?
Also I understand that the something in the brain remark might come over as judgemental, but that is in no way my intent.
I don’t believe myself but I don’t judge people who do (neither people who don’t, obviously).