Because the percentages of Mormons, Roman Catholics, and adherents of non-Christian faiths have remained steady over that same time, the rising number of "nones" has occurred as Protestants have decreased, their numbers shrinking from 57 percent to 47 percent from 2000 to 2016.

"Therefore, there are fewer Protestants of any kind in the American population today, and the pool of those who identify with a specific Protestant denomination is smaller," the Gallup report explains.

Other than Catholics or Mormons, American Christians "increasingly put themselves into a non-denominational category rather than identifying with a specific denomination such as Baptist, Methodist, Lutheran, Episcopalian." Yet others call themselves "Christian" with no qualifications.

The percentage of Christians who do not name a specific denomination has nearly doubled, from 9 percent to 17 percent in the past 16 years.

For those who do claim a specific denomination, Baptists continue to be the largest Protestant group, with 10 percent of Americans identifying as "Baptists" and an additional 3 percent identifying as "Southern Baptists."

The Gallup researchers noted that measuring the religious identification among Protestants poses some challenges, especially given that hundreds of Protestant denominations exist today.

"This allows Protestants to easily switch between denominations, with the result that many Protestants are not as fixed in their religious identity as would be the case for Catholics, Jews or those from other religious traditions," the report reads.

Even so, Gallup's consistent research process has allowed a general comparison of trends in religious identity to occur across time.

"These trends indicate that, while many Americans remain religious in a broad sense and may continue to seek spiritual guidance and community experience, a formal structure in which to do so has become less important."