OP here to give a little more info for everyone who would like some more back ground on where I got this information if the professionals want to do more digging. This book written by a professor of religion and sociology and Penn State claims that "In 1776, fewer than 1 in 5 Americans were active in church affairs." The data for the rest of the chart is contained in that book. Is this accurate information?

Keep in mind that "church membership" doesn't mean "attends a church." Drawing from David Walker Howe's What Hath God Wrought which discusses the situation during the Second Great Awakening, church membership was a lengthy process and not something undertaken by every Christian.

If Finke and Stark are using it that way, then they have some methodology issues.