Christians could fall to as little as a third of the US population over the next 50 years.
The Pew Research Center forecasts that by 2070 half of all Americans will have no religion.
Currently 64% identify as Christians and 30% have no faith.
Muslims, Jews, Hindus and Buddhists could in total double to one in every eight Americans.
The researchers stress their conclusions are based on projections from present data and mathematical models.
They point out that with so many variables their outlook is not a guaranteed prediction of the future.
The latest census in Australia found 44% of the population identifying as Christian and 39% having no religion.
On current trends, those with no faith will soon outnumber believers, with the gap growing each year.