The Pew Research Centre surveyed more than 35,000 Americans and found that adults over the age of 18 who describe themselves as Christians have dropped by almost eight percentage points in seven years - down to 70.6 per cent last year.
It said: "Over the same period, the percentage of Americans who are religiously unaffiliated - describing themselves as atheist, agnostic or "nothing in particular" - has jumped more than six points, from 16.1% to 22.8%.
"And the share of Americans who identify with non-Christian faiths also has inched up, rising 1.2 percentage points, from 4.7% in 2007 to 5.9% in 2014."
It also shows that Christians are a more diverse group, reflecting the growing Latino population.
Marriage between religions is also on the rise: "Among Americans who have gotten married since 2010, nearly four-in-ten (39%) report that they are in religiously mixed marriages, compared with 19% among those who got married before 1960."
This rise is thought to be linked to the growth of the population who do not identify with any faith.
The U.S government census does not ask Americans about their faith and so there are no official statistics on faith in the country.
