The study by Pew Research found that just under 65 per cent of Americans say they are Christian, while those identifying as religiously unaffiliated has risen to over 26 per cent.
The figures represent a 12 per cent drop in the Americans identifying as Christians now compared to 10 years ago.
Those describing themselves as atheists has doubled in the last decade from 2 per cent to 4 per cent, while agnostics now stand at 5 per cent of US adults, up from 3 per cent in 2009.
The decline in Christian affiliation is most notable among young adults.
A majority those born between 1928 and 1945 describe themselves as Christians (84 per cent), as do three-quarters of Baby Boomers (76 per cent). By contrast, only half of millennial's 49 per cent describe themselves as Christians.
"The changes underway in the American religious landscape are broad-based," Pew said.