Roughly one-third of all Protestants were either unable or unwilling to describe their specific denominational affiliation; respondents, for instance, described themselves as "just a Baptist" or "just a Methodist." Protestants account for roughly half of the adult population and nearly two-in-three (65%) Christians in the United States, but American Protestantism is very diverse. It encompasses more than a dozen major denominational families, such as Baptists, Methodists, Lutherans and Pentecostals, all with unique beliefs, practices and histories. These denominational families, in turn, are composed of a host of different denominations.
Source: Pew Research Center