I think the prevailing answer is lack of evidence, with things like suffering / death and stuff as a close second.

We all arrive atheists, most of us are then inducted by others into their cults, some of us escape.

One does not lose faith, one re-discovers one’s own agency.

I wasn't brought up in an overtly religious household, although I went to Sunday School for a short while.

Christianity, and / or the stories I was told via either Sunday School or other interactions with religious folks, never made any sense to me. It all sounded like fiction. It didn't match my sense of 'the world'.

So I never really lost faith, I never really had it in the first place, but any attempts to convince me into the fold didn't feel like they were describing reality.

Someone once attempted to recruit me into selling Amway (a pyramid scheme / Multi-Level Marketing scam, like Herbalife), and it felt similarly creepy to the way some religious folks have tried to describe their faith to me. That spidey-sense feeling that something is off.

Don't get me wrong, there are good people who consider themselves religious, but they seem to be the more 'casual believers'. A friend of mine has basically said that they literally enjoy the community, with the implication that the actual 'belief' part is incidental and matters far less. This kinda fits with my belief that religious organisations are a good thing if they're providing a service to those who need it; and there are plenty of people that need community and acceptance.

Never had it, the concept of "God" always ridiculous and authoritarian to me. I really like the narrative in the book Sapiens, identifying the "Cognitive Revolution" as the main distinguishing characteristic of this particular collection of ape species, which then lead to the competition of narratives, of which religions are one kind, taking place within that cognitive capability.

That said, have really enjoyed most of the humanist aspects of the Pope Leo AI Encyclical and also enjoyed Francis' AI essay, and I am coming to understand the perspective of those who value religious-sourced narrative for its philosophical posture. These narratives are evolving as human understanding evolves, still with lots of ridiculous contradictions (prohibition of abortion among the most blatant), but there is depth of thought around valuing humans and human work and so forth that is a pleasant contrast to the predatory power hungry totalitarian selfish nihilism of most AI/tech and political leadership.

I never had much religious faith to begin with, despite growing up in the Bible Belt and having some devoutly religious family members. I was just sort of always a natural skeptic, and somebody who was drawn to the hard sciences, and wanted to see, feel, hear, measure, test, experiment, etc. before believing things. Nothing about religious beliefs really checked any of those boxes for me.

I spent most of the time from my mid-teens (give or take a bit) to sometime in my 30's referring to myself as agnostic or maybe "weak atheist" since I can't prove there is no deity / deities. And a very slight thought that there was some merit to the "irreducible complexity" argument also weighed on that. Then I read Dawkins' The Selfish Gene and that satisfied me that the "irreducible complexity" argument doesn't hold up. Since then I've generally referred to myself as an atheist if asked about my religious beliefs. In some contexts I might say "non believer" or just defer from answering, to avoid starting arguments.

I grew up a middle American white atheist. As an intellectual, it was obvious those who practiced “religion” as conventional American Christians were hypocrites and fools.

American Christianity to my young mind was little more than a social club.

And then I traveled the world.

I came to know Godhead through the line of Mahadeva. That is to say, India and Shiva revealed to me something of the human spirit and how we as Man relate to God.

In the decades since, I have reformulated these conceptions without the dressings of any particular religion, Hinduism for instance.

Here is the divine order as I have come to account:

Supreme Divine Aspect

Totality of All

Universal Potential

Godhead

Manifest form

Truth

Emergent being

Self

I think the atheist and those of spiritual ascent may both agree that “universal potential” and “Godhead” may be synonymous, and find some common ground.

As for what face or story or interpretation of Godhead one comes to recognize or conceptualize I like to say that is it not Godhead who is the deceiver, rather Man who deceives himself.

Existence is divine. What one makes of it is one’s own path.

The terrible burden of free will is that existence is savage, and one must “chasten” their God, for they love their God, and they must perish by the wrath of their God (to paraphrase Nietzsche's Zarathustra.)

I do not have faith. I am one of principles and devotion.

Putting me in a private christian school in first grade turned me militant agnostic by fourth grade. I could tell none of the teachers really understood or believed in anything. I did not just lose faith in Christianity by then but rather in the school system and I learned to distrust everyone and that was just the start of my training and awakening.

I grew up in the Midwest. My parents dabbled with Jehovah's Witness when I was 7. I think even then I thought the concept of an all-knowing, all-powerful being that was never seen or heard seemed very odd. Now throw in no Christmas? No thank you!

Critical thinking and reasoning.

Reading the Bible and actually studying it carefully.

Then studying various other religions.

Then studying human evolutionary psychology and paleontology, as well as various regional histories.

Then it all made a LOT more sense.