In 2007, 78.4% of US adults identified with Christian groups. That percentage has now fallen to 70.6%.

Pew’s Religious Landscape Study also found that the number of Americans with no religious affiliation has risen from 16.1% in 2007 to 22.8% today.

‘The major trends seen in American religion since 2007 – the decline of Christians and rise of the “nones” – have occurred in some form across many demographic groups,’ the report said.

But despite the overall number of Christians declining, the number of evangelical Protestants has increased by 2 million over the past seven years.

The report said evangelicals ‘are gaining new converts at a greater rate than they are losing people who were raised in the tradition’.

Dr Ed Stetzer, executive director of LifeWay Research, said the results show that ‘nominal’ Christianity is dying. ‘The nominals are becoming the nones, yet the convictional are remaining committed’ he wrote.

‘What we see from Pew is not the death-knell of Christianity, but another indication that Christianity in America is being refined.

‘The cultural cost of calling yourself “Christian” is starting to outweigh the cultural benefit, so those who do not identify as a “Christian” according to their convictions are starting to identify as “nones” because it’s more culturally savvy.’

The research also showed an increase in ethnic diversity within Christianity as a whole.

The share of Americans who identify with non-Christian faiths, such as Islam and Hinduism, has grown modestly in recent years, from 4.7% in 2007 to 5.9% in 2014.