"In 2015, 24 percent of Americans identify as Catholic, 50 percent as Protestant or as members of another non-Catholic Christian religion, and 2 percent as Mormon," the report said.
U.S. citizens self-identifying as Christian remained high in 2015 at 75.2 percent of the population.
In 2014, 75.7 percent did so - a decline within the statistical margin of error - leading Gallup to conclude this: "America remains a predominantly Christian nation, with three-quarters of all adults identifying with a Christian faith, and with over 90 percent Christian representation among those who say they are a member of any kind of religion."
The decline in self-identifying U.S. Christians is more notable over time, trending downward from 80.1 percent in 2008 and more than 95 percent in the 1950s.
This 5.1 percent decrease from 2008 to 2015 has been mirrored by a 5 percent increase in the "nones" - persons who do not claim any religious affiliation - over the same period.
Gallup called this "a major religious trend" currently underway in the U.S.
U.S. citizens self-identifying with a non-Christian faith tradition remained steady - moving from 5.3 percent in 2008 to 5.1 percent in 2015 (a change within the margin of error).
"Despite these changes, America remains a predominantly Christian nation, and with 94 percent of those who identify with a religion saying they are Christian," Gallup commented, noting that Catholics, Protestants, Mormons and non-denominational Christians are included within their Christian designation.
The report added, "In 2015, 24 percent of Americans identify as Catholic, 50 percent as Protestant or as members of another non-Catholic Christian religion, and 2 percent as Mormon."
Christian identification increased with age - with 62 percent of the 18-24 demographic doing so and peaking at 89 percent among 80- to 84-year-olds.
"One key to the future of Christian representation in the U.S. population will be shifts in the religious identification of today's youngest cohorts," Gallup noted. "Traditionally, Americans have become more likely to identify with a religion as they age through their 30s and 40s and get married and have children. If this pattern does not occur in the same way it has in the past, the percentage of Christians nationwide will likely continue to shrink."
The full report is available here.