Fewer than half of American adults define themselves as Protestant, marking the first time the faith group has decreased significantly below 50 percent, according to a new study from the Pew Research Center's Forum on Religion & Public Life.

While the Protestant portion of the population has declined, the number of Americans who do not identify with any religion -- also known as "nones" -- is rising, the study shows.

One-in-five adults -- about 46 million -- have no religious affiliation: That group includes more than 13 million atheists and agnostics and nearly 33 million adults who describe themselves as having no particular religious affiliation.

Here's more from Pew detailing the results of the study:

To read the full study, check out http://www.pewforum.org/