The one argument that might make a hardline atheist stop and think...

Aggressive atheists are a bit like exotic animals – I know they exist but I havn't really come across them. I've seen them on TV, heard them debate on the radio and read their invective in the newspapers. Most of the atheists I know are fairly mild types – they don't believe in God, they may even think faith is silly, but for the most part they're cordial or even friendly and interested when discussing the topic.

This last weekend, though, I had an encounter with a lesser-spotted species which left me surprised and a bit taken aback. But eventually the encounter left me more confident in something. Christianity actually works.

I was at a social event and was introduced to a friend of a friend. We'd spent a pleasant couple of minutes chatting when he asked what I do for a living. I told him I write about Christian faith and other religious issues.

At this he unleashed a string of accusations about Christianity and other faiths which I'd never seen in such a concentrated form before. He accused all religions of being completely fictitious, of being the cause of all violence in the world, of being inherently divisive and abusive and much more besides. It was like meeting an aggressive Twitter conversation in real life.

While I struggled to get a word in edgeways for a while, I eventually managed to suggest some alternative points of view and some factual corrections to his points. I pointed out that actually, the person who'd probably killed the most other humans in history was Stalin – the head of an atheist regime (or maybe it was Mao – also head of an atheist state).

I noted that the sheer beauty of where we were standing (in a forest with trees towering over us) was a good reason to believe in a god. I suggested that the extraordinarily finely-tuned nature of the universe for human life was an indication that atheism was also a faith position. I was about to turn to the moral argument, when the conversation took another turn.

"What about South Africa?" I asked. "You say faith is inherently violent and abusive, but there's an example of Christianity in action." I went on to suggest that the peace and reconciliation process which followed the end of apartheid was explicitly based on Christian principles. The process itself wasn't perfect and, of course, South Africa has large social and racial problems to this day. But without the process of forgiveness and reconciliation, there is little doubt that a horrendous civil war could have ensued.

When I'd finished, my opponent conceded that this was, in fact, a good point. He then returned to his litany of complaints and we parted later on after a handshake and an agreement that we were unlikely to agree any time soon.

What stayed with me after the conversation was the one small concession that my interlocutor had made. Christian principles put into action on a large scale had broadly worked and been the key to avoiding large scale bloodshed and disorder. The core Christian concepts of forgiveness and reconciliation had won a major victory for human rights and relative peace.

But I would say that, wouldn't I? I'm a Christian writer on a Christian site, talking to a mainly Christian audience. There isn't any way of backing up what I'm saying. Or is there?

Days after my confrontation in the forest I was listening to the excellent NPR podcast Invisibilia. The show looks at the invisible forces that act upon our lives – especially in the field of human behaviour. Last week's episode, entitled Flip The Script, is an investigation into so-called 'noncomplematary behaviour'. It's about the way in which individuals and societies can respond to wrongdoing in a surprising way. Rather than responding with the usual iron fist, which escalates a cycle of mistrust, isolation and even violence, the show examined various situations in which understanding and even forgiveness have been shown.

Here's a segment: "the basic idea is that people naturally mirror each other. So when someone is hostile to you, you are typically hostile back. Warmth begets warmth. And breaking this pattern – say, being really warm to somebody after they've been incredibly hostile to you – that is noncomplementary behavior." The script goes on, "The reason, for example, that we admire people like Gandhi and Martin Luther King Jr. is because they were able to maintain a sort of warmth and integrity in the face of people who were being cruel to them."

I'd go a step further than the Invisibilia presenters and suggest that not only do Dr King and Ghandi show warmth and integrity – they are modelling Christian values. King spoke in explicitly Christian terms. But even Ghandi, who wasn't a Christian, learned many of his non-violent resistence ideas in conversation with Leo Tolstoy – one of the preeminent Christian pacifists of the 19th and 20th Centuries.

The podcast goes on to describe an incredible programme which deals with former ISIS volunteers in Denmark who are being reintegrated into the community rather than harshly punished. It is a compelling story.

Here's the thing – Christianity works. We're used to the idea that individual testimonies are a strong witness to the truth and power of God. But we're less familiar with the use of larger scale episodes of forgiveness and grace. While scientific, philosophical, cultural and other arguments for God can be powerful and persuasive – they may not win over sceptics. But the track record of the principles of Christianity applied and writ large may just persuade even a hardened atheist that there is something to this ancient faith after all...

Follow Andy Walton on Twitter @waltonandy