Washington University Professor of Sociology and Comparative Religion Rodney Stark used Church records and Census data to come up with his findings. His book, "The Churching of America" based the 17% figure on narrowly defined data of church attendance. Stark is an atheist and I find this source cited a lot on atheism websites.
"Church membership" is a very iffy proxy for belief, particularly early on in America when it was difficult for many to get to church. But "membership" is not the issue. Belief is>
""Church membership" is a very iffy proxy for belief, particularly early on in America when it was difficult for many to get to church."
That's very true. Nominal Christians are in much higher numbers than are recorded in church membership records.
I remember the morning, while I was just starting USAF Basic Training, that we were asked our religious affiliations, for the purpose of making our dogtags. A few recruits had a ready answer. Catholic, Presbyterian, Methodist, or, in my case, atheist. Many, however, didn't know what to answer.
The drill sergeant, in his drill sergeantly way, said to the confused ones, "Are you a Catholic or a Jew?" When they answered in the negative, he told them they were Protestants, and that's what got inscribed on their dogtags.
Were those recruits Christians? Perhaps, in a nominal sort of way, but they were classified as Protestants in any case, and counted among the Christians in the USAF.
My declaration of atheism was another matter, altogether. I finally got it on my dogtags, but it was a battle, I'll tell you. I may even have set some sort of precedent back in 1965. Now, it's no problem. The military even has a symbol for atheism.
My point is that many people call themselves Christians by default, and that may well have been true back in the early day of our nation. It's impossible to say.
I know this, though: If every church in every community were absolutely full every Sunday morning, attendees would represent a small minority of the population. The rest are nominal Christians. If you ask them if they are Christians, they'll probably say yes. If you dig a little deeper, you'll discover that they are only nominal Christians, who give little thought to that name.