I'll focus on your title and ignore the body of the text because many other people are already focused on that. I once considered myself a christian, so why don't I believe it any longer?
I kept wondering: "how can they possibly know that" when they were talking about what heaven is supposed to be like. So epistemology was a huge problem to me.
christian beliefs are really elaborate. I can't fathom how the people around me can believe in 1) people being raised from the dead 2) saint Anthony helps you find lost items 3) wine turned into blood (local miracle) 4) a hosti didn't get desolved by stomach accid (supposed miracle in Amsterdam) I fail to see the justification for any such "additional"-beliefs. There is no way you can get from "there is a god" to "it is the Christian god" to "you must also believe these additional beliefs". These supposed miracles happened hundreds of years ago, even if they provided real wine and an intact cracker it wouldn't prove anything. Meanwhile my uncle who believes that he can speak with pigs and has 'magnetic powers' has a lot of family members who completely believe him and have 'witnessed' him doing his magic and are completely sure he has magical powers. I'm sure anyone here won't believe in his magical powers, so a book claiming whatever about a person living centuries ago is not something I can possibly trust, even with numerous witnesses, witnesses are not reliable.
there is plenty of confirmation bias going on: for example Saint Anthony, I was told by my parents (cafeteria catholics mind you) that "you must pray to St. Anthony until you find your lost item". This ofcourse will work by definition, this of course didn't sway my parent's minds to them it still works. You could easily pray to a spoon and it will be equally as effective. You can pray for 90 years, you only stop when you have found your lost item. Since most people will pray for car keys and what not any object you pray to will have a high succesrate. Other instances "See that rainbow noticed the beautiful weather during the funeral? It was god's doing!" never mind all other funerals with bad weather.
I thought I felt the holy spirit or however you want to call it. I also felt the presence of a boogeyman living underneath my staircase, apparently when I no longer believe in the entity I no longer feel 'any presence' just like the boogeyman disappeared so did God. Of course some christians will claim that my heart hardened or something like that.
the neurophysicist in me just cannot accept "bodyless minds" "atemporal minds" "mind consisting of 1 or 0 thing(s) (divine simplicity)" or a "time independent mind that still interacts with time dependent minds eventhough those time dependent minds have free will". I've heard people claim that free will and all knowing atemporal minds aren't contradictory, but I don't think that if you were a rat in a maze and I were changing the walls you still have some significant 'free will' to speak of. I would be your puppeteer.
tldr; there is just so many things that are impossible to me or lack so much evidence that I just cannot possibly accept it.
I think that it isn't brought up enough that Atheism itself IS a worldview, and as such has its own philosophical implications.
My atheism is simply a consequence of my skepticism, I view the world as a skeptic not as an atheist.
