Thanks for the correction.
But one needs not be a member of a church in order to consider himsel/herself a Christian.
As an example, I belong to two Catholic organizations, yet am not a member of any church. I was an altar boy and take part in Catholic fundraisings, support the Holy See and its prelate, yet I'm not currently a formal member of a church. Still, when I filled out our 2000 Census, I describe myself as a Christian. So, in our census numbers it matters not whether one is an active member of a church in order to qualify as a Christian.
The same applies here to our early colonists. And that is, although most people were not members of a church, their offical religion, their traditions, their country of origin, their ceremony and their culture was Christian-based. Religion gives rise to culture and these early people's culture was clearly Christian.
True enough. But, unlike Catholicism, and especially in the 17-18th centuries, Protestant churches (especially the Calvinist traditions) did not consider those who were not actual church members as "Christians" other than in a particularly cultural sense. Membership not so much meaning a particular church at the moment, but having been accepted as a member by a Church through (typically) adult profession of faith. (Think of their origins as non-conformists to either Rome or the Church of England) There were fairly significant controversies in New England about this in the early 18th century.