Britain's most prestigious universities could almost have been designed to make sure that less-advantaged students become fundamentalist Christians.

David was the first person in his family to attend Oxford. He was the first to experience highly formal meals and Latin rituals, making aristocratic friends from public schools and somehow putting together two essays a week. And he was also the first person in his family (who are Anglican for the purposes of weddings) to have a religious experience. One Sunday, having gone to church with some Christian friends, he "just felt that God was there for me. I just knew it. I was kind of weeping ... but I was smiling on the inside! I can't describe what it felt like!" By the time he graduated, he was "just ... fundamentally different as a person".

For David and many of his non-religious friends, what happened to him was shocking. The change in him was massive. No more heavy drinking, no premarital sex. Instead, there was regular church attendance and daily Bible study. But David's transformation is hardly unique. Prestigious universities such as Oxford, Cambridge and Durham are pretty much built to leave students wondering who they are and where they belong. The institutions do everything they can to smash the certainties of place, social class, religion ... all the ways we use to make sense of the world.

This means that the Government's push to get more students from less well-off backgrounds into Oxbridge has some interesting and unforeseen consequences. It is these undergraduates, confronted with life at institutions such as Oxford, who are the most likely to end up with their sense of identity in tatters. And this means that the student-run evangelical groups operating at many UK universities become very attractive, offering the possibility of the world making sense once again - and making sense for eternity.

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For the average former public-school pupil, the ancient universities do not present a major identity crisis. Attending Oxford offers no great change in social status when you're already pretty high up the social ladder and your father and grandfather went there. The arcane rituals aren't so daunting when you've known about them - and even been preparing for them - for years.

Living away from "home" will represent no major change. If you've been to public school, you may have lived away from home for years and have very little identity in terms of a city or town where you're "from". And if you're privately educated, you are far more likely to have been on an identity-challenging gap year to some far-flung corner of South America. Indeed, you are far more likely to have come across evangelical Christianity because it thrives among the upper-middle and upper classes. But whereas Oxford is no big challenge for the average public-school pupil, for a student like David with non-religious parents and a comprehensive-school background, things are not so easy.

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The Oxford experience is not unlike the tribal rites of initiation, assiduously studied by anthropologists, that serve to raise the status of children into fully fledged adults able to lead the tribe. Graduates from prestigious universities are the ones expected to run our tribe. And just as with the Ndembu of Zambia, they reach this higher status through suffering. Oxford cuts off its neophytes from the tribe so it can break them down and fashion them anew, indoctrinated with new ideas and ways of thinking. This has already happened to most public-school pupils, but for many state-school students, there's a lot more raising - and breaking down - to be done.

David certainly found university difficult. "I hated it at first. I was so overjoyed to have got in but after a few weeks I hated everything about it. I just wanted to go home," he recalls.

Oxford took him away from everything that his sense of self was built on: his home town, his family and friends. It placed him in a college with people he would have had little to do with before: students from affluent, privately educated backgrounds; students from all around the country; and a few who were fervent Christians who quickly joined the university's Christian Union.

"Their views made me angry. I'd never met anyone with views like theirs. I thought, 'How could anyone believe this!' We're meant to be among the most intelligent people in the country!" But after a while, David recalls, it got him thinking. "Nothing made sense to me any more. But gradually what they were saying just seemed to make more and more sense."

In such a stressful environment, an incredibly friendly group that seems to have all the answers can be very attractive. Around half of the undergraduates at the University of Oxford are from state schools, but they make up about two thirds of the members of the Oxford Inter-Collegiate Christian Union (OICCU). These state-school pupils are far more likely to have "become a Christian" while at university. Frequently, they credit the OICCU - which runs three meetings a week and a number of "enjoyable events which aim to share the good news of Jesus Christ with every student", according to David Meryon, last year's OICCU president - with a central role in "saving" them.

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Other students attest that the experience of attending an elite university can be just as distressing for those who already consider themselves Christians.

Naomi, a former member of the Durham University Christian Union and originally from Shropshire, admits: "I lived in a kind of bubble of Christianity before I went to university. I knew non-Christians from school, and I had non-Christian friends. But living with non-Christians (at Durham) meant having 'deep' conversations well into the night. I'd never had to defend what I believed from people who were tearing it apart."

Naomi had always considered herself a Christian, she says. "But it was at university that I realised I wasn't - I realised I was going to hell, basically!"

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For Christians, as for non-religious students, elite universities challenge their sense of who they are. Accordingly, Christian Unions serve to "preserve" their faith by a creating a strong Christian community in which members must sign a "declaration of faith" before being accepted.

According to Meryon: "It ensures we're focused on what unites us around the Gospel, on the non-negotiables of what Christianity is." This fortress mentality also helps to keep the group pure, with the result that many Christians who attend elite universities leave more dedicated to their faith than they were when they arrived.

"I'd always thought of myself as a Christian," Naomi says. "But the Christian Union made me realise that I wasn't. I hadn't given myself to Jesus." After Jesus appeared to Naomi in a vision one evening in college, she says, she reaffirmed her faith in Christianity - and realised that her non-Christian friends will go to hell.

Research in the US has found that Christian students who attend Christian colleges tend to come out with more liberal attitudes as they mature and become more educated, but that Christian students attending secular Ivy League universities become more fundamentalist as their faith is challenged. At most higher education institutions in the Netherlands - which, like many post-1992 institutions in the UK, draw their students largely from the local area - Christian student societies tend to be more liberal, and religious conversions are rare. In the largely egalitarian Finnish higher education system, strongly religious 19-year-olds are likely to leave university as rather liberal Christian 26-year-olds.

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Elite UK universities such as Oxford can be psychologically violent places, and undergraduates for whom attending Oxford represents a significant change in status are likely to find their time there the most violent of all. This trauma is likely to leave them the most susceptible to the blandishments of evangelical groups that promise to make the world a warm and comfortable place once again. Such students may leave Oxford to find a sought-after job, but they are far more likely to have met Jesus while they are there. And he may stay with them all their lives. l