Many factors prevented me from believing in a a supernatural (not real) god.

Desire to base my life on truth not fiction.

Lack of evidence to support any supernatural god exists.

Church teachings that contradict reality demonstrate faith (belief without evidence) asserted as facts is both dishonest and should not be considered a methodology to determine truth (things that can be demonstrated). Figured that out early grade school.

Lack of need or positive outcome in imagining a god is real.

Understanding that reality free of superstition holds great beauty that is diminished by fantasy based make-believe.

A combination of things...

First, being raised around "spiritual" people that we're consistently unkind, negative to children, duplicitous, and unhappy. The proof that something wasn't right before apparent quickly.

Finding that kinder people tended to be more moderate in religious thought.

Having constant ear pain as a child and doing a crazy amount of prayer, but a doctor did it in a few days.

Finding the most interesting music was made by people that didn't find religion useful or that it needed to be central.

Having a mom that worked with a Christian chiropractor that ended up injuring her back severely. A regular doctor had her fixed in less than a week.

At the end of the day, the proof is in the pudding. Things that worked better than 50% came about by people working hard and learning. When you see such consistent positive results from non religion at the same costs, it just kind of sorts itself out.

I can't figure what causes anyone to believe in god. Not believing just seems so obvious.

I had already questioned it but once I read the Bible front to back that did for me for good!

Common, or perhaps more accurately uncommon, sense.

Critical thinking, it's simply not possible.

Wow. What a great question. The bible is what did it for me. And organized religion. It all feels so archaic. So many gender roles and fear based following. I also think critical thought has caused me to not believe in God as well.

My ex kept shoving it down my throat.

I simply read he Bible

Never believed in god(s). Taught by my parents to ask questions and find my own answers. There seems to be a big lean to monotheism, especially the Abrahamic three. Invisible friends are a bit odd in adults, I think.

My Roman Catholic schooling. I found the story of Adam and Eve quite offensive even at an early age. Then the whole forced routine of confessing sins. Flash forward to my passion for ancient history and anthropology. That ended all notions of religious beliefs.

My god beliefs failed to accurately explain or predict outcomes in my life ... that was the proximal impetus for me to begin to apply critical thinking to the human condition. Of course once you do that, it's game up.

I never really bought into religion. My dad was a high school science teacher, so phenomena were explained scientifically. I spent a lot of time at odds with almost everyone in my rural community because of it, which certainly didn't make religion more appealing. And the more attention I paid to the stories and explanations the more paradoxes and contradictions arose and asking questions just led to circular logic arguments that began and ended with God, and the nature of religious faith demands an absolute rejection of all answers that aren't 'because its God's will.'

Well if it's God's will, and questioning God's will is blasphemy, isn't prayer blasphemy? You're asking God to change his omniscient Grand Design to suit your tiny insignificant human desires.

In the end, the absolute insistence AGAINST logic, curiosity, and critical thinking spiked any possible interest in the prospect.

The hypocrisy of religion. Raised Catholic, and when the prohibition of eating meat on friday was lifted what happens to all those suffering in hell for eating meat?. Do they get an out of hell card and all is forgiven with restitution. Also it took them almost 400 years to admit that Galileo was right.

All this info I gleaned over time and now mostly condensed here. It opened the eyes of a very religious friend.