Christianity in America is declining and the country is becoming more secular, according to a new report from the respected Pew Research Center.

While seven out of 10 Americans still identify with a religion, the number professing 'no religion' has grown to 56 million, making the 'nones' group the second largest behind evangelicals. Protestant Christians now comprise only 46.5 per cent of what was once a predominantly Protestant country.

The findings are based on the 2014 Religious Landscape Study of 35,000 people. A previous survey was conducted in 2007 and the 2014 study was designed to be comparable.

It showed that the decline in Christian affiliation was particularly pronounced among young adults, but was noticeable across all ages, all races and people with different levels of education. It represents a fall of nearly eight percentage points in the number describing themselves as Christians, from 78.4 per cent to 70.6 per cent. Over the same period, the percentage of Americans who describe themselves as atheist, agnostic or "nothing in particular" has jumped more than six points, from 16.1 per cent to 22.8 per cent. Those identifying with non-Christian faiths have also increased, from 4.7 per cent in 2007 to 5.9 per cent in 2014.

The drop in the Christian share of the population has been driven mainly by declines among mainline Protestants and Catholics, the research shows. Each traditions has shrunk by approximately three percentage points since 2007. Evangelical Protestants have also declined, by around one point as a share of the population, though they have increased in absolute numbers by around 2 million.

As the total number of Christians is declining, they are also getting older. The average age of mainline Protestants is 52 and that of Catholics 49, both an increase since 2007; by contrast, the average age of unaffiliated adults is actually falling, down to 36 from 38.

Allowing for margins of error, Pew says, "it appears that the number of Christian adults in the US has shrunk by somewhere between 2.8 million and 7.8 million". The most startling figure, however, is the increase in 'nones' – a rise of 19 million since 2007.

The survey found that a large factor in the overall decline of Christianity has been the number of people who have left the faith altogether rather than simply switching to a different expression of it. While some who have been brought up without faith later find it, Pew says: "For every person who has joined a religion after having been raised unaffiliated, there are more than four people who have become religious 'nones' after having been raised in some religion. This 1:4 ratio is an important factor in the growth of the unaffiliated population."