Via Gallup:
Americans have become less likely to identify with an official or formal religion in recent decades, and nowhere is this more evident than in the dwindling percentage who identify with a specific Protestant denomination. In 2000, 50% of Americans identified with a specific denomination; by 2016 that figure had dropped to 30%.
This shrinking proportion of Americans who identify with specific Protestant denominations is the result of two trends.
First, an increasing percentage of Americans are “nones,” saying they don’t have a specific religious identity of any kind. Since the percentages of Catholics, Mormons and those who identify with a non-Christian religion have stayed roughly the same over time, this “rise of the nones” — from 10% in 2000 to 20% in 2016 — has generally been accompanied by an associated decrease in the broad category of Protestants, whose numbers shrank from 57% to 47%. 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.
Secondly, Americans who identify as Christians other than Catholics or Mormons increasingly put themselves into a non-denominational category rather than identifying with a specific denomination such as Baptist, Methodist, Lutheran, Episcopalian and so forth. Others simply identify their religion as Christian without any reference to Protestantism or a specific Protestant denomination. As a result of these trends, the percentage of Americans who identify with a specific Protestant denomination has dropped from 50% in 2000 to 30% in 2016, while Christians who don’t name a specific religion or denomination have doubled in number, from 9% to 17%.