In a fast-changing world, the churches still preserve their traditional language; who else but a churchman is nowadays described as "trendy"? The stylised language with which clergymen attack each other should warn us against taking their words at face value when they discuss the decline in Christian belief. On Friday, the Archdeacon of York, George Austin, blamed Lord Runcie, the former Archbishop of Canterbury, for a decline in church services because of his trendy liberalism; and Lord Runcie himself yesterday blamed "happy-clappy, huggy-and-feely worship".

Both were joining in a tradition much older than the innovations they condemn. A decline in Christian belief and practice is one of the most important facts of this century, at least in northern Europe. It is a much larger phenomenon than the 3 per cent annual drop in Anglican attendances which sparked off the latest round of ecclesiastical backbiting. The explanations for it are likely to go far deeper than liturgical fashions.

In 1833, Thomas Arnold, the headmaster of Rugby school, wrote: "The Church of England as it now stands, no human power can save." Thirty years later, his son, the poet Matthew, heard "the melancholy long withdrawing roar" of faith on Dover beach. Between those periods, Christianity actually grew more popular and more influential in England; but both men were, in a sense, prophetic. Throughout this century, Christian belief has declined in Western Europe. In North America it still prospers. In the Third World and in Eastern Europe, some forms of Christianity are growing at tremendous speed. But in the UK, almost all are failing.

The Methodists have been warned by their own statisticians that they face extinction in the next century, due to a lack of young people. The Roman Catholic population is declining even faster than the Anglican. The Church of England thought until last week that it had arrested its long decline and could look forward to a modest but satisfying rise in membership. Those expectations are the only justification for calling a loss of 36,000 members out of more than a million a "crisis".

The first instinct of clergymen confronted with such figures is to blame the opposition. In the Church of England, most of the opposition is other parts of the Church of England. So traditionalists blame liberals for bowing to secular pressures, and liberals blame traditionalists for demanding belief in miracles repugnant both to common sense and morality. Yet variations in the international statistics on church attendance may not reflect on changing styles of worship so much as on a shift in the social base of religious belief to poorer and less educated parts of the world. In mainstream churches all around the world, theological liberalism now is as unfashionable as socialism and optimism in the secular world.

Perhaps these changes of fashion are connected. Maybe the enemy of religious belief is neither liberalism nor conservatism but some quite extraneous measure, such as affluence or unemployment. There are certainly statistics which suggest that a sociological approach may tell us more than any theological explanation. The two denominations which are in the worst trouble in England are the Methodists and the Roman Catholics. In both churches, left and right blame each other. But in both churches there are powerful sociological reasons for the decline which transcend the theological ones. Methodism and Irish working-class Catholicism were both anchored in distinctive subcultures which have been wiped out economically. The countryside, where Methodism flourished, has been depopulated, and few members of the working classes work in Catholic strongholds such as Liverpool. They have become either unemployed or suburban.

These changes are devastating because religious belief never exists in isolation. It is always embedded in particular subcultures and forms of society. When a particular society and pattern of religious observance dies, then the belief tends to die, too. Re-establishing it in new cultures can be hugely difficult. Often the result is simply ridiculous, and seems to produce only pop culture cut with holy water.

Those were the kind of suspicions that lay behind Lord Runcie's attack on "happy clappy" services. Speaking in Cambridge yesterday to a sympathetic student audience, Lord Runcie attempted to explain himself: "They may be getting more young people in, but if young people can only respond to an ecclesiastical version of a bar on Saturday night, while the middle- aged demand a sort of health farm, then we mustn't pander to that. We must just say the culture is against us; and accept a drop in numbers."

Lord Runcie's willingness to accept quality over quantity, as he sees it, is very different to the activist approach of his successor. Dr George Carey believes in trying things. He has encouraged alternative services, which try to sanctify dance culture, even after the scandalous collapse of the best-known experiment on those lines, the Nine O'Clock Service in Sheffield. He takes for granted the sort of modern "choruses" based on catchy, amplified tunes which have largely replaced hymns in evangelical churches.

In fact, a dislike and suspicion of these choruses unites Lord Runcie with the Rave in the Nave types whom some newspapers have assumed he is attacking. One of the main reasons the Nine O'Clock service was founded in the mid-Eighties seems to have been to get away from the liturgical use of Eagles tunes from the Seventies in its parent church. Alternative worship groups, talking among themselves on the Internet, worry constantly about anchoring their services in popular culture without having them dragged down by it. They, too, are afraid that the only future for the church may be in niche marketing, with different services targeted at different congregations, which turn out to have less and less in common with each other. But they don't know what to do about the problem: nor, I think, does any Archbishop, and possibly not even the Archdeacon of York.