As a staunch atheist with a visceral dislike of being preached to, the idea of being sent to Greenbelt, England's biggest "post-evangelical" Christian festival, didn't have much appeal. I picked the events less likely to send me into an apoplectic state of secular revulsion at religion: my goal was to learn and explore without feeling like a prey to be set straight at all costs by the evangelical left.

I arrived on Saturday morning and first stopped at the underground venue showcasing non-stop rock bands. Rock'n'roll would help me ease my way in, I thought. The melodic-emo rock band Arcane Roots was playing an early set for the festival younger crowd, and while I expected to suffer through cringe-worthy Christian rock songs, I was relieved to hear really decent songs instead, even though the audience didn't seem to be able to get in the mood. I also dropped by Greenbelt's Folk club, in which musicians jammed together in improvisation. The session was lovely, if only a bit lonely: I was the only member in the audience to not be able to sing along, clap hands and praise Jesus. Amen.

At midday I sat in the sun-kissed grass and listened to Tasmin Omond, who was giving a speech on the Church and its relationship with environmentalists. Tasmin is the founder of Climate Rush and an ex-Plane Stupid activist who in 2008 got arrested for scaling the Houses of Parliament to protest against Heathrow's third runway. She also flirted with the idea of becoming a priest, and in no uncertain terms she voiced her frustration at the disconnect between religious groups' official line on climate change (read: almost nonexistent) and the everyday activism of church-goers.

The stands at Greenbelt were extremely environmentally and ethically conscious. They made for a fantastic display of bleeding-heart Christian liberalism, peppered by slogans in support of Palestine, a show produced by Christian Aid, and flyers denouncing CEOs' paychecks. The food and craft market was all crocs, fluorescent plastic crucifixes, 100% hemp clothes, recycled plastic items, tie-dye t-shirts and fair trade or organic food products.

Tasmin Omond, Greenbelt merchandise, a drawing workshop and the Jesus Arms pub at Greenbelt Photograph: Jessica Reed/guardian.co.uk

It is only after lunchtime that my day took a more personal and interesting turn. I joined a session called Serum, which aimed to facilitate theological debate for participants who "wouldn't necessarily call themselves Christian". Fully expecting to be lectured about my beliefs, I got seated at a table and told to imagine what my feelings would be if I was to be The Creator and had to judge people after their deaths. My fellow participants were, from what I could guess, all believers, and it made for a really frustrating but ultimately fascinating debate, during which we all considered forgiveness, human v godly judgement, free will and whether or not we would like to punish bad people if given the chance ("Oh yes, I would definitely like to zap Pol Pot", whispered the middle-aged joyous lady sitting opposite me).

Thanks to a few ground rules ("no one has definite answers" and "respect one another"), the discussion went smoothly and I was left wondering why such groups weren't more popular - the exercise permitted me to relate to people in spite of the massive gap separating us. The experience was both humbling and powerful, especially as I had to remind myself that the debate's outcome wasn't to make a point to win, but to listen to ideas and belief system different from mine.

By mid-afternoon I headed towards the workshop centers to attend a Quaker meeting. Because they rely so little on a prescribed creed and conservative rules, Quakers have usually benefited from a good reputation amongst secularists and atheists alike, and I was no exception. After reading Rosemary Hartill's excellent Cif piece on the recent Quaker decision to recognise same-sex marriages, I read more about their faith – and even learned that some Friends call themselves Quaker atheists.

And so I joined the meeting with an open mind, and quickly found myself sitting as part of a silent worship. The first few minutes were unnerving as the audience settled, and I couldn't help but feel like a small child forced to observe silence at school: I had to keep immature giggles in check and stop myself from observing everyone in the room. I soon stopped however, and found myself submerge by a sense of peace I can only describe as relief, for it is too seldom that one is given the permission to sit with strangers in silence, without behavioural expectations, and allowed to look inward in synchronicity with others.

Twenty minutes went on and the practice was starting to get hold of me, when someone finally got up to speak. I found myself completely taken aback when my eyes started to water. The simplicity with which he picked his words, and the silent but respectful silence which followed, allowing each of us to consider what had been said, was not only moving but also touched on something pure and true – a moment unlike I had ever witnessed before.

I took a short walk after the meeting to reflect on what had just happened, almost ready to declare myself quite tolerant of "post-evangelism": I hadn't been preached at once, nobody had pitied me when I announced my lack of faith, and I couldn't really say that my personal leftist politics weren't in line with the festival's.

I decided to finish the day by attending a film workshop, in which two presenters had chosen to talk about what God had said to them through movies. After showing film extracts, the audience would be shown choice quotes on screens, aligned side by side with quotes from the Bible – and repeat those all together.

The exercise turned out to be entertaining, especially when the audience repeated the Stars Wars mantra hand in hand with a biblical verse ("May the force be with you").

The young presenter carried on to project clips from Whale Rider, in which a young Maori girl fights to fulfill a destiny her grandfather refuses to recognize. He later explained that he had seen the movie in Leicester Square in the midst of a personal crisis – during which he doubted his job as a youth worker, feeling as if his work was in vain. But, he said, God showed him the way through the movie, telling him he had to find the strength to carry on, that we had to fight society's growing secularism and help more young people "find the way and find God". My stomach turned and my French blood, which always fought for laïcité, began to boil.

"So this is what it's all about", I thought. "A social worker showing no understanding, no dialogue – secularism and spiritual differences are to be fought against, and children subtly coerced into believing?"

This kind of discourse, I couldn't deal with. I turned around at once, and left.