In times past, conversions were like earthquakes, momentous events that happened far away, to other people. Now they are like nose jobs or heart valve operations: everybody knows somebody who's had one.
It's not hard to see why. The certainties of political ideologies are fizzling out, ties that bind people to places, creeds and identities slacken, allowing them to make drastic and repeated revisions to their allegiances and beliefs. It is now possible to seek salvation in a vast and widening array of spiritual panaceas, everything from aromatherapy to Zen, vegetarianism to fundamentalism. The opportunities and temptations to convert are multiplying.
In the case of converts to Catholicism, the rich and famous sniff the way the wind is blowing, mull the optimum timing for an announcement, and send out feelers to the "prime social priest" (Clark's words), Father Michael Seed, regarding the procedure.
Out of sight of the media, unnumbered thousands do something similar, but without fanfare. The high-profile conversions to Catholicism are part of a trend that is affecting many religions: Clare Hershman of the Buddhist Society says many of those battering on the doors of the churches and temples are "just fed up en masse - terrified and hopeless and looking for answers".
The Church of England's crisis over the ordination of women provoked the most conspicuous exodus of prominent members in the church's history. At least five bishops, 300 priests, and 25,000 lay members converted to Rome in the past five years.
The evangelical sects, in Britain as elsewhere, are soaking up huge numbers of recruits, many moving from more conservative denominations. Ian Cotton, author of The Hallelujah Revolution, claims that today there are 400 million charismatic Christians worldwide, twice the number there were 10 years ago.
The Islamic revival has attracted millions of new adherents around the world, Jemima Goldsmith being the most prominent recent example. Soka Gakkai, the evangelistic Japanese Buddhist sect, has made inroads among rock and jazz musicians, while thousands are attracted to the austerity and conundrums of Zen Buddhism.
Fundamentalist Protestant sects in the United States constitute a formidable political force, while their kin in Latin America have outstripped Catholics in numbers and fervour. Christianity is much the most potent new force in countries such as China and South Korea, where vast new churches rise above the skyline of major cities.
At the heart of these developments is the experience of conversion. Whether you are coming from Anglicanism or cannibalism, from addiction to crack or belief in the coming proletarian revolution, the act of embracing a different faith, possibly one you had regarded with suspicion if not enmity, has the same psychic ingredients: one's beliefs were misguided if not downright wrong; the future will be better because one has found one's true spiritual home.
"We have done what we ought not to have done and we have not done what we ought to have done," as the prayer book puts it, "and there is no health in us. But thou O Lord have mercy upon us ..."
Clare Hershman sees Alcoholics Anonymous's methods for weaning people off drink as the archetype of conversion. "People who have made a complete mess of their confusion and who have turned to addiction to comfort themselves get into the idea of not exploring their feelings, the idea of restraining their impulses for the sake of higher achievement. Signing up with AA is an act of handing oneself over to a higher self. Because most people need a higher self, whether you call it Buddha, God, or whatever."
This shift towards more traditional values may be a reaction to the growth since the Fifties of more liberal attitudes towards sex and marriage, according to Stuart Hall, professor of sociology at the Open University. "There's been lots of hype about it, but there is solid evidence of this secularisation, too, of a shift to secular values. Such a shift is always accompanied by an attempt to turn back, to return to traditional values. There is also a connection with the failure of political ideologies that promised people long-term satisfaction. The political parties are not in the business of overarching visions. So religion is the only thing left that promises to change the world.
"The rise of religion in the United States is striking, with its aspiration to political power, driven by a simple-minded materialism. The growth of Protestant sects in Latin America is due to the failure of Marxism and the revolution theology that went with that. Instead they are going for 'Protestantism of the American Way' - buying into a certain kind of modernity. It's a heady brew."
Converts to Islam are drawn by opposite properties. "It offers a tight- knit, cohesive community," says Dr Phillip Louis, author of Islamic Britain, "which is a challenge and a rebuke to certain sorts of individualistic Christianity. For many Afro-Caribbeans, suffering alienation from English society, Islam offers a more hospitable home than Christianity, and a much more male culture." The same qualities have persuaded numerous "Anglo- Catholic ex-public schoolboys nostalgic for Christendom", in Louis's words, to plump for Sufism.
But the path of the convert, whatever his destination, is not strewn with roses. "I'm sorry ... and shocked at the malice of some in the Conservative Party, gleefully abetted by sections of the media," Alan Howarth wrote, one week after his defection to Labour. "Every variety of fabrication and innuendo, systematically fed from Tory sources, has been used in the attempt to discredit me."
Conversion is an affair that arouses passions: apostasy, treachery, excommunication, outer darkness - the darkest, direst concepts in our language attach to the act of spurning that which has nourished you and cleaving to the new. Switching political parties is a relatively trivial matter. Paul Johnson, who travelled in the opposite direction to Howarth more than two decades ago, says: "Parties are only a means to an end; if a party fails, one can abandon it without regret. Your religion is quite different: it has a bearing on all eternity." But whether the issue is one's immortal soul or the result of the next election, the emotions aroused are often the same.
The joy of those who welcome the convert is as intense as the disgust of those left behind. Labour's delight at the arrival of Howarth was unconfined. Likewise Cardinal Hume, in an unguarded moment, rejoiced in the defections from the Church of England. "This could be the moment of grace," he told The Tablet, "it could be the conversion of England for which we have prayed all these years."
For those left behind, the departure of the apostate can have disastrous effects on morale: it is not merely the pain of rejection that hurts, but also the doubt which defection casts on the whole edifice of belief that has been abandoned. If one person goes, what is to prevent a host of others from doing the same - what is to prevent the building itself from collapsing?
Likewise, the boost in morale for the group the convert joins can seem wildly excessive. The arrival in the Labour Party of one Tory ex-junior minister was greeted with a jubilation that would be more appropriate on a victorious election night.
For the convert, the dominant emotion, according to the employment minister Ann Widdecombe, is "an overwhelming feeling of relief". She misses certain things about her old church. "Church bells - Catholic churches are not allowed to ring bells to summon congregations; hymns sung with Protestant fervour. But it's only nostalgia, absolutely no regrets. I was very, very disillusioned for a long time with the liberalism, the tolerance of anything and everything, the refusal to take moral stands."
She left in November 1992, but was not received into the Catholic church until April 1993. "I made up my mind in March. In between, I was resolving questions of doctrine. I was required to declare that 'I believe that all the Catholic Church teaches is revealed truth.' It took four months to decide that I did."
Charles Moore, the Daily Telegraph's new editor, left the Church of England for similar reasons, though 10 years earlier. "The Catholic church has become the only intellectually credible church in England," he said. "It stands for something coherent. The Church of England attracted intellectuals until 30 years ago - people like TS Eliot and CS Lewis - but not any more. It used to be liberal in the good sense of tolerant and relaxed. Now it's liberal in the bad sense: irreligious, sloppy, just sort of confused.
"In the Catholic church I still feel a bit of an alien - but an alien like a tourist, excited by the new place you're in."
That is a phrase that well defines the experience of the convert - whether Howarth, with his new view of the Commons, Charles Moore relishing confession, or the reformed alcoholic, tasting the novel sensations of restraint and communal approval. All are excited by the new place they're in. But what happens when the excitement palls, when you've had enough of the sea and the sun of your new spiritual resort?
Moore declares stoutly, "I don't have any expectation or desire to stop being a Catholic." He admits, none the less, that for him, the Catholic church is not "coming home" but "more like being on the right road; the life of belief is a journey".
But having made the convert's move once, having left the faith of one's birth, how is one to be sure that the journey is over? In her time, Ann Widdecombe has been an evangelical Protestant, an agnostic, a middle-of- the-road Anglican and a Catholic. Now, she says, "I have arrived. I have found Peter. I should have done this a long time ago." But who is to say that some day her feet will not again start itching? The priest who converted her, Fr Seed, has himself in his short life (he is 37) passed through the Salvation Army and several varieties of Baptists before arriving at Peter's gate. It is easy to imagine him defecting again, this time perhaps to something exotically millenarian.
The converts themselves may strenuously deny it, but conversion, with its attendant passions, becomes a habit. "Ratting" and "re-ratting", as Churchill described his repeated journeys across the Commons, is the modern disease. It is the nomadism of modern man; the realisation in the guts that one's true spiritual home, whether in religion or politics, is no more than a myth - followed by a feverish quest for a better imitation of it.