More than half of those surveyed, 52.5 percent, said they are Protestants, 23.6 percent Catholic and 1.9 percent Mormon.
The 78 percent who describe themselves as Christians has dropped only slightly since 1951, when 82 percent told Gallup they were Christian.
Fifteen percent say they are atheist, agnostic or do not identify with one religious faith, a group that has increased from near zero in 1951. But 92 percent said in a separate survey that they believe in God.
Only 1.6 percent said they are Jewish, 0.5 percent Muslim and 2.4 percent members of another non-Christian religion.
More than 80 percent said religion is an important part of their lives.
The data come from the Gallup Daily tracking survey Jan. 2-Nov. 30, which had a sample of 327,244 adults. The margin of error is 1 percentage point.