Although people in America were (with some exceptions) quite religious throughout most of its history I’ve noted that the majority of adult Americans were not church members even in the 1700s and 1800s. In Puritan New England of course you had to have prove of salvation to be a church member but afterwards why wasn’t a sizeable portion of the American people church members? While Deism was certainly popular, certainly most Americans were Christian in some form or other.
What is the question exactly? Church membership doesn’t have a strict definition across Christian denominations. For some like the Catholic or Amish church, there is a formal process. For others like the Episcopal church, there isn’t. I go to an Episcopal church but I am not listed on any official registry. I was baptised in a Methodist church as a child and there aren’t really any other requirements beyond that to self-identify as Episcopalian and a member of that congregation but there isn’t a way for an outsider to know that unless I tell them.
Where did you get your stats and what definition of church member did they use? I am not sure I believe it but it would be interesting to see where that claim could come from.
Page 80 of this book discusses it briefly. Basically as you suspected, formal church membership was a poor proxy for actual church attendance in the colonies. The book hypothesizes this is due to the fact that unlike in Europe, the churches weren’t dominant political institutions that one needed to formally join to participate in various aspects of community life.
I suspect actually getting yourself in a Church was non-trivial for places in the colonies where population density was low and travel difficult as well. So even the Church attendence estimates probably undercount the number of people that considered themselves active Christians in the colonies, at least early on.
The hsitory of the really old churches here (we have a few that go back to the 18th Century) suggests that they started as groups of families that got together to pray. As years went by they would eventually find a minister, then a few more years would go by and they’d have enough families to build a special building and hold regular services.
Even then, record-keeping was probably pretty lax. A minister might keep track of baptisms, deaths and marriages, but as for actual membership? If you were there, you were a member.