One Third of Americans Affiliate with Evangelical Churches

30% is massive, if it is accurate. Lots of people say on forms that they are part of a church when really they don't attend. But that is mostly in the mainline ones: One Third of Americans Affiliate with Evangelical Churches - Desiring God:

Those who argue that a new American landscape is emerging—one in which the conservative evangelicalism of the past few decades is losing numbers and influence—are simply ignoring the data.

Membership in the mainline denominations declined 49 percent from 1960 to 2000; membership in evangelical denominations increased 156 percent in the same period.

Fully one-third of Americans (approximately 100 million) affiliate with an evangelical Protestant congregation. Indeed, evangelicals remain the numerically dominant religious tradition in the United States.