The received wisdom among people who hold their religion dear is that mixed-faith marriages always end in tears.
But a revolution in attitudes is under way, spearheaded by a rabbi in Maidenhead. Jonathan Romain runs seminars and counselling sessions for mixed-faith couples and plans to publish a guide to help them. His work has been criticised by some members of the Jewish establishment but has been taken up by Jewish centres around the country and also has attracted the interest of the Church of England: last June he was invited to address the House of Bishops Conference. Over the past five years he has helped more than 1,500 couples.
These are just the tip of the iceberg, however. Nowadays, 70 per cent of Catholic and 50 per cent of Jewish marriages are with outsiders. According to Mr Romain, Orthodox Jews are just as likely to marry out as Progressive Jews. The only exceptions are the ultra-observant, such as Hasidic Jews.
'The problem has been seriously underestimated,' he says. 'When I started this work I was regarded as totally off the wall and heretical. But now it's seen as an important issue affecting a major part of the religious population.
'A lot of people are starting to realise that just because a Jewish person has married a non-Jew, it doesn't make them any less Jewish, and the same goes for other religions. Many people in mixed-faith relationships still value their religious roots and want to share them with their children.'
In Mr Romain's experience, religious differences generally cause problems. But he believes these can be overcome, and even become a strength, if couples are prepared to tackle their differences and discuss everything before making a commitment.
'Many mixed-faith couples feel their relationship has a better chance of success because they've had to work to sort everything out beforehand - from their ideas about home life to the children's education,' he says. 'It's rather like marriage guidance in advance.
'But most people aren't desperately religious, they consider themselves lapsed. They tend to underestimate their religious feelings until later. Then, particularly when they become parents, they often relate back to childhood memories of religious traditions.'
This presumably explains why many lapsed C of E couples send their children to Sunday school. The religion we were brought up in, however nominally, and however much we reject it, colours our attitudes. 'There are certain things that people take for granted, that they see as having less to do with religion than family tradition, but which the other person sees as controversial,' Mr Romain says. 'Such as visiting the in-laws at Christmas, which one partner refuses because he or she doesn't observe Christmas.'
The biggest conflicts invariably arise when children are born. Mr Romain has lost count of the calls he has received from new mothers distraught because their partners insist that the baby should or should not be circumcised.
The next hurdle is the children's religious upbringing. There are two common options: bringing them up in both faiths or banning religion altogether and allowing children to decide when they are older. Mr Romain remains unconvinced by either approach. 'Because this is a Christian society, it's hard to give an equal balance between two faiths. And having no religion is a mistake, because you can't choose from a vacuum. It works best when parents divide education and identity, so that the child has roots in the religion of one parent and is taught about the religion of the other.'
The predictable crisis points in mixed-faith relationships are cycle-of-life celebrations, starting with the wedding. Mr Romain has pioneered a practical compromise: a registry office ceremony, followed by a DIY service at home, led by a relative, with music and readings from both religions.
But even if the couple sail through these occasions, their families rarely do. Mr Romain has encountered a depressing catalogue of prejudice: the Protestant couple who, although lapsed, refused to attend their son's wedding to a Catholic; the Jewish grandmother who has never met her grandchildren; the Muslim who is terrified of telling his family about his Christian girlfriend.
Mr Romain is always willing to talk through parental distress, and tries to make the son or daughter understand the views of the older generation. 'Parents have every right, whatever age their children, to say, 'I personally don't like this.' But they've got to accept the child's decision. I try to make them see that what's really important is the quality of their relationship. If religion has any value, it's to draw people together.'
Yet Mr Romain's personal convictions seem surprisingly hard-line: he admits that he would be disappointed if any of his four sons married out of the faith. 'I'm not in favour of mixed-faith marriages,' he says. 'I believe it's better to marry within your own faith, whatever that faith is. But I recognise the right of other people to choose. I feel torn between my loyalty to the Jewish community and my desire to help individual couples. I'm walking a tightrope. But it's a matter of tackling reality. Over the past five years a lot of people have come to realise that this is the only way forward.'
Jonathan Romain's guide for couples in mixed-faith relationships is available from RSGB, The Sternberg Centre, 80 East End Road, London N3 2SY (pounds 1, including postage). Details of counselling and support groups are available from Dot Swaro at the same address.