Only 6% of adults in Britain are practising Christians and very few of them decided to become Christians during adulthood, according to a new survey.

ComRes carried out the study in an attempt to map churchgoing and religious practice on behalf of the Church of England. It published its figures on Tuesday. It defined 'practising Christians' as people who read or listen to the Bible at least once a week, pray at least once a week and attend a church service at least once a month.

The responses suggested that most people who call themselves Christian are 'cultural Christians'. Only 42% of Christians who took part said they ever read the Bible; 26% went to Church more than once or twice a year; and 28% called themselves 'an active Christian who follows Jesus'.

The survey also revealed how young many Christians are when they first adopt the term. A total of 85% said they became Christians during childhood, and 64% said it happened between the age of zero and four. Just 24 of the 8,150 respondents said they decided to become Christian after the age of 44.

Stephen Evans, the NSS's campaigns director, said those figures "make clear why the Church of England is desperate to expand its role in schools".

The survey suggested that 51% of British adults call themselves Christian, 56% of whom were Anglican. The proportion of people with no religion was 42%. Last week data from the highly-respected British Social Attitudes (BSA) survey suggested the level of non-belief was even higher: 53% of British people said they had 'no religion'. Just 15% of people – and only 3% of those aged 18-24 – called themselves Anglican in that study.

The discrepancies between the two surveys are likely to be the result of different methodology. The ComRes survey asked 'To which of the following religious groups do you consider yourself to be a member?' and listed religions. The BSA survey asked respondents: 'Do you regard yourself as belonging to any particular religion?' If they then said yes, they were asked which one, without being given a list of religions.

Mr Evans said the new figures provided "another reminder that Britain is not a Christian country".

"Politicians who continue to support the establishment of the Church of England and giving the Church a role in state education look out of touch.

"But it is also worth stressing that the case for secularism would remain overwhelming whatever surveys such as this said. Even in highly religious countries, it is in everyone's interests to resist the imposition of fundamentalism and prevent religion from gaining political power."

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