As Protestants decline, those with no religion gain:
Persons who identify themselves as Protestant in religious affiliation now comprise only 48 percent of the country's population.
Just under 20 percent of Americans say they have no religious denominational affiliation at all, a larger number than belong to any Protestant denomination. Among people under age 30, a third disdain any such affiliation.
See my 2010 essay, "Why 'Spirituality' Rather Than Religion?"