What is the most popular religion in the U.S.?

[Henry Tackman]

Christianity is the largest religion in the United States, more than three-quarters of Americans are Christians. A little more than half of the U.S are Protestants, about 23 percent as Catholic and about 2 percent as Mormon.

source: https://en.wikipedia.org/wiki/Religion_in_the_United_States

[Dakota Schaefer]

"The Christian share of the U.S. population is declining, while the number of U.S. adults who do not identify with any organized religion is growing, according to an extensive new survey by the Pew Research Center."

Source: http://www.pewforum.org/2015/05/12/americas-changing-religious-landscape/