Christians have no monopoly on forgiveness. That's the view of Marina Cantacuzino, founder and director of The Forgiveness project, a secular organisation that describes its aims as helping people to share stories and experiences in order to build understanding, encourage reflection and enable people to reconcile with pain and trauma.

That Cantacuzino wants to free the act of forgiveness between human beings from what some have called the 'straitjacket' of religion is not that surprising. Most thinking Christians will recognise that our faith (or faith itself) is not the only possible motivator to forgive. What is surprising is not that Cantacuzino is no evangelical, but that she would not even describe herself as "evangelical about forgiveness".

"I find forgiveness fascinating," she tells me at Greenbelt Festival, where she has been speaking. "I don't think it's always the answer, I think it's fine not to forgive if you don't want to."

It's a strange statement, she acknowledges, from someone who has spent the last 11 years of her life working in the field of forgiveness, but she maintains: "I'm not sure I'd call myself evangelical for forgiveness. I'm evangelical for sharing stories. And I do believe that listening to the pain of the other – the enemy, the person who has hurt you – will often bring about compassion, empathy and create a more peaceful world."

But beyond that, for Cantacuzino, forgiveness is more complex and nuanced than those who talk about it most might have us believe. "Everyone's got their own definition," she says. "It's a word no one can agree on. It's pretty slippery." And of course she's right. Every cell group and Bible study must at some point have the conversation about what constitutes forgiveness and how far it should be taken – how seriously we should try to emulate Jesus' own forgiveness. And that's just the laity. Clergy and theologians have debated its nature for centuries. At Greenbelt on a previous day, Giles Fraser has expressed the view that forgiveness is not a feeling, but a decision not to take revenge. Cantacuzino is not sure she fully agrees.

What she is certain about is that compelling, pressurising or pushing people to forgive is counter-productive. "Forgiveness is a gift and you destroy its power as a gift if you make it a duty," she says, recounting a conversation she once had with a senior official in South Africa's Truth and Reconciliation Commission – often held up as an example of how repentance and forgiveness can heal historic wounds – who was glad that individual forgiveness was never made a compulsory part of that process. "It's a very personal thing, forgiveness. I don't think it should be doled out by administrations. At a community level perhaps, but not from on high."

From re-traumatising victims of abuse by pressurising them to forgive their abusers to the covering up of wrongdoing and blaming serious illness on unforgiveness, the dangers in pushing too hard for forgiveness should be obvious. An organisation with a name like The Forgiveness Project must clearly walk a fine line. Canatcuzino sees it primarily as a place to get people to consider forgiveness – and that that in itself can be valuable. "'Consider This' is what I often think the project should be called," she says. "We hold a very nuanced position at The Forgiveness Project. We're a place of inquiry, discussion and exploration. I think if forgiveness becomes an obligation, it becomes a tyranny."

For most mainstream Christians, this position poses a problem. Cantacuzino herself is not convinced that Christianity emphasises forgiveness more than any other religion, though she does admit some frustration at the fact that many people assume that hers is a Christian organisation. And the reason for that is simple. Christianity does put an extraordinary amount of emphasis on the importance of forgiveness. From Christ's words as he was crucified and his teachings on loving enemies, turning the other cheek and going the extra mile to Old Testament concepts like Jubilee and any understanding of Jesus' death or our relationship with God that focuses on taking away sins.

The Lord's prayer emphasises forgiveness – sometimes translated as a forgiveness of debts, sometimes sins, but forgiveness. The concept is everywhere in our faith and you have to do some impressive intellectual gymnastics to get to an understanding of the Christian life as not involving some instruction or expectation that we will forgive those who wrong us.

But, as Cantacuzino points out, the area is fraught with complexity. She is uncomfortable with forgiveness, "in any case where you feel pushed towards it. That might be you're pushing yourself, or it might be an administration or government or family are pushing you towards it," she says. "Then I think it will create all sorts of untold problems. You have to feel the rage first, and then the sadness, and only then might you find peace of mind." Trying to fast-track the process of forgiveness prematurely can clearly be problematic. And, she says, even more tricky within families, particularly where someone has been lost. "To forgive can be quite an isolating journey," she says. But clergy, she recognises, "maybe have a default position of encouraging people to forgive".

So how are we to stand with those who want or need to forgive, without pressuring them to enter that process before they are ready, but also without pretending that our faith will not sometimes make demands of us that the world may not understand and that may go against our natural inclinations?

Perhaps it is to create space for people to talk and share their experiences of hurt in the way that The Forgiveness Project does – space and opportunity to hear other people's stories of pain, and sometimes hope. More importantly, it may be time for the Church – from those who fervently believe in forgiveness to those who see it within strict limits of repentance or restitution and those who understand some wrongs to be beyond its reach – to acknowledge that forgiveness is a complex beast.

"I probably had a more simplistic view of forgiveness when I started out on this," Cantacuzino tells me. "I was going to say I understand it more [now]. I wonder if I understand it less. Because all the stories I've collected and the more than 140 people I've interviewed, every one shows you a different aspect of forgiveness, a different journey, a different thought. I am fascinated by the complexity of it and aspect of uncertainty around it."

But after hearing so many stories of pain and abuse, after experiencing so many different responses to the process of forgiveness, does this secular forgiveness expert (a term she will almost certainly detest) still value its process and power? Or has she become jaded?

"I still believe it is a route out of stuckness," she says. "If you cannot stop thinking about your best friend who did something to hurt you, or former partner or partner or a random attack from a stranger, whatever it is, I think forgiveness is a route out."

Cantacuzino's favourite definition of forgiveness is a poetic one. Mark Twain's, in fact: "Forgiveness is the fragrance that the violet sheds on the heel that has crushed it."

Her own definition is perhaps more helpful: "I think forgiveness is about not allowing the pain of the past to poison the path of the future."