"SEAN made a Christian commitment at a Crusader camp in his early teens, and when he went off to study at Nottingham he attended the Christian Union, at least for the first year," Rob, who is married to Cathy, says. They have two children: Sarah, aged 35, a committed Christian and an active member of a pioneering congregation in west London; and Sean, 33, who today describes himself as an agnostic.

"Friendships and sport were very important to him, and eventually he admitted that the CU people were 'boring' in comparison with his other friends," Rob says. "At the same time, the then pastor of the church he had been attending sporadically, for whom he had a high regard, resigned suddenly in questionable circumstances, and we believe that this may have had a significant impact on Sean's view of the importance of faith in life. He soon went on to let us know that he was no longer a believing Christian.

"When Sean and Sarah were young, we had regular family times spent praying together and reading the Bible. We were members of a lively church, with great children's and youth work, and we did our best to encourage open discussion and questioning. Of course we wish Sean had found faith for himself, as Sarah did, but there are no guarantees."

Rob and Cathy's experience is far from unique. Reliable statistics are difficult to come by, since a person's faith is not fixed and cannot be empirically measured, but, in 2005, a study funded by the Economic and Social Research Council suggested that half of children raised by two religious parents reject church as adults; and research conducted by the Barna Group between 2007 and 2011 suggests that nearly three in five young Christians disconnected permanently, or for an extended period of time, from church life after the age of 15. To state the obvious, belief does not pass through the placenta.

"Of course, your hope is that they'll want to go to church," Joanne, whose daughter Leah is an atheist, says, "but they have to make up their own minds. We try to keep supporting her and loving her no matter what, and we are genuinely proud of her - she has so many qualities we can celebrate and affirm." Joanne's philosophical approach seems to be rare. More common are feelings of fear, guilt, and shame that can put family relationships under immense pressure.

MICHELLE OBAMA famously said that a mother is only ever as happy as her saddest child. The ultimate goal of parenthood may well be to launch independent adults into the world, but a parent's sense of well-being will almost always be entwined with theirs, to some degree. If someone walks away from the faith he or she was were raised in, the ones who did the raising will inevitably be affected.

Graham's son Ben recently told him he no longer considered himself to be a Christian. Graham is still reeling: "My overwhelming emotion is one of guilt. I keep going over the past, over things I have said or things I have failed to do, and asking myself 'What have I done wrong?'"

Other parents in the same situation say that they feel ashamed, and wonder if their fellow Christians are judging them. Rachel's son hasn't come to church with the rest of the family since he was 14 years of age. "When I look at other families with all their children in the pew with them, the fact that Tom chooses not to come with us makes me feel rubbish," she says. "Comments from other parents make me feel like we've messed up in our parenting."

The chairman of the charity Care for the Family, Rob Parsons, counsels against this in his book Bringing Home the Prodigals (Hodder & Stoughton 2003): "So many parents are carrying a heavy load of guilt they have no right to bear. That's not to say they have been perfect parents. They have just been parents - parents who have given this task their very best effort."

When children do not identify themselves as Christians, among the other emotions that this can produce in their parents is a profound sense of rejection. "I know Elliott has rejected our values and not us," Megan says, "but sometimes it feels as if the two are intertwined; our faith is so much part of our identity."

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IN SOME instances, of course, there is a degree of rejection involved. Children watch their parents and other Christians as they grow up, and perhaps do not like what they see. "Based on outside observations of Christians, there's no way I would want to become one of them," Dan Kimball says in They Like Jesus But Not the Church: Insights from emerging generations (Zondervan, 2007). "I wouldn't want to become an angry, judgemental, right-wing, finger-pointing person, [and] I wouldn't be saying that out of rebellion against God or the Church."

But often, instead of rejecting their parents - or even God - adolescents and young adults find that it is a particular Christian sub-culture, or straitjacketed orthodoxy, that fails to fit their sense of how the world works.

And, sometimes, the word "rejection" is itself an overstatement. In Disappointed With Jesus? Why do so many young people give up on God? (Monarch Books, 2010), Gavin Calver writes: "The young person feels trapped [by religion, tradition, and church], but their hope cannot be taken away. Hope that there's something else; hope that within all the tradition, legalism, and hypocrisy that they encounter within the Church perhaps there is some truth in the gospel."

Feelings of guilt, shame, and rejection are amplified by certain underlying assumptions. Perhaps the chief of these is the belief that Jesus is exclusively "the way, the truth, and the life". Many Christian parents want their children to embrace what they see as not just the only way to peace with God, but the only way to fulfilment and a meaningful life.

Those who believe that anyone who does not follow Jesus will endure eternal punishment are understandably even more anguished about the faith choices of their children. Their certainty may set an agenda that can threaten healthy family relationships.

Sally is not concerned about her sons' eternal destiny. "Neither of my sons are churchgoers, and actually I am proud that I raised sons who feel free to go their own way," she says. "When I see grown families all sitting there together in a service, it makes me wonder what manipulation has gone on - whether they've really been allowed to grow up and become their own people."

ANOTHER belief is that God promises the faithful that their children will mirror their parents, as in the book of Proverbs: "Start children off in the way they should go, and when they are old they will not depart from it." When the children do depart from the way, some parents become angry with God for breaking his promise.

Others assume that, since God is perfect, they must be the ones to have failed. Addressing this issue, the psychiatrist John White writes: "Many parents get hurt because they find false hope in the Bible. I do not mean that the Bible is unreliable, but that, in their concern for their children, parents may read the Bible through magic spectacles."

From the point of view of the adult child, things can be just as complex and painful. While a teenager may turn against God as a way of separating from his parents, in order to find his or her own identity, loss of faith in the long term is unlikely to have anything to do with a desire to hurt or rebel against anyone.

"I lost my faith when I started university, on being challenged to question my beliefs," James, an accountant in his 30s, says. "I discovered that I had no reasons of my own for believing. I've been very aware of how my parents felt - and still feel - about my apostasy, and have been at pains to explain to them that it was an intellectual realisation that made belief impossible, rather than me rejecting God, or them. I'm not sure if they've ever fully understood this."

In many ways, it could be easier for a child to avoid this issue. Life might be much easier, basking in parental approval, with a loving God looking on, securing a benign eternity. It takes courage to follow where conviction leads.

ALFIE has four siblings, all of whom are committed Christians. It has been hard for him to reach the point where he feels he can be honest about the spiritual conclusions he has come to. For years, he found it easier to allow them to assume that he was still on board. He attended church, and said the right things. When he finally came clean, he felt a sense of release, but it has not been easy.

"Admitting that I am not a Christian has made me feel separate from the rest of the family. I am massively outside of something which they all share, and there is nothing I can do about that.

"After taking the plunge, and telling them all I did not and could not believe, it was hard. For a few years, they could not get their heads around it. There was a lot of tension around small things, from not going to church with them if I was visiting to bigger things like moving in with my girlfriend, and being unmarried - shock, horror! I think they hoped that I would 'come round' at some point; and I was made to feel like I was hugely letting them and the family down.

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"From my point of view, it is my decision about a religion, and in no way a reflection on them - or, to be fair, to do with them. I am aware that it affects them, but I don't think it's fair to make me feel like I've let anyone down - but I do, hugely."

HANNAH, a vicar's daughter from the Midlands, has never been made to feel that she has let her parents down. Nevertheless, she resents it that they view themselves as failures, because neither she nor her sister is a Christian. "They feel they weren't good parents to their girls as teens. They wanted to hold on tight to us, to push us forcefully into belief. They think they ended up pushing us away. I wish they didn't blame themselves, because that undermines the fact that we can think and decide individually."

No parents with strong religious convictions are likely to stop hoping that those they care for will come to see things in the same way. But while some cannot let go of a sense that they are somehow responsible for the faith choices of their children, others see their position as less central.

"While Jonty drifted away - more of a lifestyle choice - Lauren is more academic, and made a decision not to continue with her faith. We haven't had the con- versation as to what was at the root of them drifting away. They might tell us one day," Caro says. "Our relationship with them is still strong, and we try hard to enter their world, and understand the things they are passionate about. It is hard, sometimes, to see other families where children are engaged with their faith, but God has spared us the risk of being proud, and thinking it's anything to do with us."

A parting of the ways over spiritual matters is not inevitable among families. But my research for Keeping Faith: Being family when belief is in question suggests that it is not at all uncommon. The families who coped best were those where relationships were conducted with mutual respect, and a willingness to accept difference.

Jo Swinney, with Katharine Hill, is the author of a new book, Keeping Faith: Being family when belief is in question, published by Scripture Union at £7.99 (CT Bookshop, £7.20); 978-1-84427-737-7. The names of the people quoted in this article have been changed.