Catholics made up 24 percent of the U.S. population in 2007 but this fell to 21 percent in 2014 as more people have abandoned the Catholic Church than any other religion in the country.
Concern is also mounting that recent clerical sex abuse cases and allegations of church-sanctioned cover-ups will lead to a further decline in numbers.
At the same time, more Catholics are converting to other religions than among any other faith, with 13 percent of adults who reside in the U.S. but were raised Catholic having migrated to Protestantism or renouncing all religion, according to the report.
Some 51 million adults in the country still identify as Catholic in America, which has over 17,000 parishes.
Caucasians make up 60 percent of American Catholics and Hispanics around 33 percent but the latter now account for the bulk of those newly drawn to the faith, the report indicated.
It said the number of Latino Catholics is expected to grow in the future while white Catholic numbers are feared to keep dwindling.
Geographically, Catholics are spread quite evenly across the country. The rate is fractionally higher in the South, however, as the religion’s center of gravity has shifted from the Midwest and Northeast, it said.
The report added that more U.S. Catholics now believe the church needs to amend some of its policies, including allowing priests to marry.
As more countries allow same-sex marriages, about half of those surveyed by Pew said they think the Catholic Church should solemnize gay and lesbian marriages.