Americans are becoming less Christian and more secular, the Pew Research Center reported based on a massive study it conducted between June and September 2014.

While the United States remains home to more Christians than any other country in the world and 7 in 10 Americans continue to identify with a denomination, the percentage of adults who describe themselves as "Christian" has dropped by nearly 8 percentage points in just seven years, investigators detailed.

During the same time period, the number of those who call themselves "atheist," "agnostic" or "nothing in particular" jumped more than six points; Muslims and Hindus have also added followers, contributing to an increase of Americans who identify with non-Christian faiths.

Evangelical Protestants made up 25.4 percent of the adult U.S. population, down from 26.3 percent in 2007, NBC News detailed. Some 20.8 percent of Americans identified with the Roman Catholic Church (down from 23.9 percent), while 14.7 percent adhered to mainline Protestant denominations (down from 18.1 percent).

Greg Smith, Pew's associate research director, told the Associated Press that his findings suggested "substantive changes" among the religiously unaffiliated and not merely a shift in how people label themselves.

Secular groups have been more successful lately in countering bias against them and in using lawsuits and lobbying efforts to keep religion out of public life, Smith added.

One of the side effects of the drop in affiliation seems to be a rise in religious intermarriage, the Pew Research Center noted.

Thirty-nine percent of Americans who have gotten married since 2010 report that they are in religiously mixed marriages; among those who got married before 1960, that figure drops to 19 percent, the report found.

Investigators also uncovered an age gap between the religiously unaffiliated -- who are comparatively young and getting younger, on average, over time -- and American religious groups, which they found are "aging."

Pew's survey was conducted in English and Spanish among a nationally representative sample of 35,071 adults interviewed by telephone, on both cellphones and landlines. It has a margin of error of plus or minus 0.6 percentage points.