The global population of non-believers is growing at more than twice the rate as the number of Christians, with Muslims catching up even faster.

Christianity is the world’s largest religion and their numbers are growing, but the proportion of the global population who is Christian is shrinking while the proportions who are Muslim and who are religiously “unaffiliated” are growing quickly.

Over the decade between 2010 and 2020 the number of Christians around the world grew by 121.6 million to reach 2.3 billion according to analysis by the Pew Research Centre in the United States, a leading group looking at global religious trends.

This growth was slower than the growth in the overall global population, meaning that the proportion of the world classed as Christian fell from 30.6 to 28.8 per cent between 2010 and 2020. Over the same period, the number of people who are “religiously unaffiliated” grew by more than twice as much, increasing by 270.1 million to reach 1.9 billion, growing from 23.3 to 24.2 per cent of the world.

This is partly explained by the fact that “large numbers of people, mostly Christians, switched out of religion” altogether, a report found. The unaffiliated group includes atheists but also counts agnostics and those who answered “no religion”, “none” or “nothing in particular” to questions about their religious beliefs.

Thanks to a younger average age and a higher birth-rate, the number of Muslims grew even faster, increasing by 346.8 million to 2 billion, increasing from 23.9 to 25.6 per cent of the population.

If current trends continued at the same pace, Christians would slip into third place globally behind unaffiliated people and Muslims by 2050.

However, Pew projections made in 2015 suggested that Christianity will still be clinging to top spot in 25 years, reaching a point in 2050 when there will be 2.8 billion Muslims compared to 2.9 billion Christians, representing 30 and 31 per cent of the global population.

Those projections from a decade ago suggested that the growth of those with no religious affiliation would slow before they overtook Christians and Muslims. However, Pew previously estimated the global population of non-believers in 2010 at 1.13 billion, but have revised that 2010 figure up to 1.63 billion, suggesting there are more religiously unaffiliated people than they had realised.

In terms of raw numbers, the UK has the ninth largest population of non-believers in the world.

Christians remain the largest group and are a majority everywhere except in the Asia-Pacific, Middle East and North African regions. Sub- Saharan Africa now has more Christians than Europe, the report said.

“Between 2010 and 2020, religiously unaffiliated people grew more than any group except Muslims, despite their demographic disadvantages of an older age structure and relatively low fertility,” said Pew, which analysed data from 2,700 sources including censuses and surveys.

Census data for England and Wales suggests that Christians still outnumber non-believers in this country, but the British Social Attitudes Survey has been showing that those with “no religion” have accounted for more than 50 per cent of the population for about a decade.

The data suggests that between 2010 and 2020, the proportion of Christians in the UK fell from 62.4 to 49.4 per cent, while non-believers saw the largest growth from 28.8 to 40.2 per cent. The proportion of Muslims grew more modestly from 5.3 to 6.4 per cent, while the proportion of Hindus grew slightly from 1.5 to 1.7 per cent. People with no religious affiliation made up a majority in ten countries in 2020, up from seven in 2010. These were China, North Korea, the Czech Republic, Hong Kong, Vietnam, Japan, the Netherlands, Uruguay, New Zealand and Macao.

In 35 countries the population of non-believers grew by five percentage points, compared with three countries where the same was true for Muslims and one, Mozambique, where it was true for Christians.