They prayed whenever they could find 15 minutes. "Preacher Man", as we called him, would read from the Bible with his tiny round glasses. It was the only book he had ever read. A dozen or so others would listen, silently praying while stroking rosaries, sitting on bare mattresses, crammed into a half-painted dorm room.

I was the outsider, a 16-year-old working on a summer custodial crew for a local college, saving money to pay for my escape from my hometown. The other employees, close to three dozen, were working to feed themselves, to feed their kids, to pay child support, to pay for the basics of life. I was the only white, everyone else was African-American.

Preacher Man tried to get me to join the prayer meetings, asking me almost daily. I declined, preferring to spend those small work breaks with some of the other guys on the crew. We would use the time to snatch a quick drink or maybe smoke a joint.

Preacher Man would question me, "What do you believe in?" I would decline to engage, out of politeness. He pressed me. Finally I broke,

I am an atheist. I don't believe in a God. I don't think the world is only 5,000 years old, I don't think Cain and Abel married their sisters!

Preacher Man's eyes narrowed. He pointed at me, "You are an APE-IEST. An APE-IEST. You going to lead a life of sin and end in hell."

Three years later I did escape my town, eventually receiving a PhD in physics, and then working on Wall Street for 20 years. A life devoted to rational thought, a life devoted to numbers and clever arguments.

During that time I counted myself an atheist and nodded in agreement as a wave of atheistic fervor swept out of the scientific community and into the media, led by Richard Dawkins.

I saw some of myself in him: quick with arguments, uneasy with emotions, comfortable with logic, able to look at any ideology or any thought process and expose the inconsistencies. We all picked on the Bible, a tome cobbled together over hundreds of years that provides so many inconsistencies. It is the skinny 85lb (35.6kg) weakling for anyone looking to flex their scientific muscles.

I eventually left my Wall Street job and started working with and photographing homeless addicts in the South Bronx. When I first walked into the Bronx I assumed I would find the same cynicism I had towards faith. If anyone seemed the perfect candidate for atheism it was the addicts who see daily how unfair, unjust, and evil the world can be.

None of them are. Rather they are some of the strongest believers I have met, steeped in a combination of Bible, superstition, and folklore.

The first addict I met was Takeesha. She was standing near the high wall of the Corpus Christi Monastery. We talked for close to an hour before I took her picture. When we finished, I asked her how she wanted to be described. She said without any pause, "As who I am. A prostitute, a mother of six, and a child of God."

Takeesha was raped by a relative when she was 11. Her mother, herself a prostitute, put Takeesha out on the streets at 13, where she has been for the last 30 years,

It's sad when it's your mother, who you trust, and she was out there with me, but you know what kept me through all that? God. Whenever I got into the car, God got into the car with me.

Sonya and Eric, heroin addicts who are homeless, have a picture of the Last Supper that moves with them. It has hung in an abandoned building, it has hung in a sewage-filled basement, and now it leans against the pole in the small space under the interstate where they live.

A picture of the Last Supper hanging in a crack house in New York. Photograph: Chris Arnade

Sarah, 15 years on the streets, wears a cross around her neck. Always. Michael, 30 years on the streets, carries a rosary in his pocket. Always. In any crack house, in the darkest buildings empty of all other furnishings, a worn Bible can be found laying flat amongst needles, caps, lighters, and crack pipes.

Takeesha and the other homeless addicts are brutalized by a system driven by a predatory economic rationalism (a term used recently by J. M. Coetzee in his essay: On Nelson Mandela). They are viewed by the public and seen by almost everyone else as losers. Just "junkie prostitutes" who live in abandoned buildings.

They have their faith because what they believe in doesn't judge them. Who am I to tell them that what they believe is irrational? Who am I to tell them the one thing that gives them hope and allows them to find some beauty in an awful world is inconsistent? I cannot tell them that there is nothing beyond this physical life. It would be cruel and pointless.

In these last three years, out from behind my computers, I have been reminded that life is not rational and that everyone makes mistakes. Or, in Biblical terms, we are all sinners.

We are all sinners. On the streets the addicts, with their daily battles and proximity to death, have come to understand this viscerally. Many successful people don't. Their sense of entitlement and emotional distance has numbed their understanding of our fallibility.

Sonya with her cross and rosary. Photograph: Chris Arnade

Soon I saw my atheism for what it is: an intellectual belief most accessible to those who have done well.

I look back at my 16-year-old self and see Preacher Man and his listeners differently. I look at the fragile women praying and see a mother working a minimum wage custodial job, trying to raise three children alone. Her children's father off drunk somewhere. I look at the teenager fingering a small cross and see a young woman, abused by a father addicted to whatever, trying to find some moments of peace. I see Preacher Man himself, living in a beat up shack without electricity, desperate to stay clean, desperate to make sense of a world that has given him little.

They found hope where they could.

I want to go back to that 16-year-old self and tell him to shut up with the "see how clever I am attitude". I want to tell him to appreciate how easy he had it, with a path out. A path to riches.

I also see Richard Dawkins differently. I see him as a grown up version of that 16-year-old kid, proud of being smart, unable to understand why anyone would believe or think differently from himself. I see a person so removed from humanity and so removed from the ambiguity of life that he finds himself judging those who think differently.

I see someone doing what he claims to hate in others. Preaching from a selfish vantage point.