For this first time, a majority of Americans no longer consider themselves to be Protestant.
The Pew Forum on Religion & Public Life reports the number of Protestant adults has fallen to 48 percent.
One of the reasons for the change is the growth in nondenominational Christians. The study does not consider them to be categorized as Protestant.
But a spike in the number of American adults who say they have no religious affiliation also contributed to the decline. That number has risen from 15 to 20 percent in the last five years.
Scholars debate whether people who say they no longer belong to a religious group should be considered secular.
Pew researchers included atheists in that category, but they also included majorities of people who say they believe in God, along with a notable minority who pray daily or consider themselves "spiritual" but not "religious."
Still, Pew found overall that most unaffiliated Americans are not actively seeking another religious home, indicating that their ties with organized religion are permanently broken.
Analysts say the broader trend could have political implications because people who describe themselves as having no religion are overwhelmingly Democrat and support abortion rights and gay marriage at a higher-rate.
The Pew analysis, conducted with PBS' "Religion & Ethics Newsweekly," is based on several surveys, including a poll of nearly 3,000 adults conducted June 28-July 9, 2012.