What is the center of Christianity in America?
Are you speaking in aggregate?... If so, then south-central is a centroid estimate.
If you're speaking of particular denominations, then you'll have to be more specific.
What do you mean by this Question ?
Are you looking for a Place , or for a Church-Organisation.
It seems to me that since there are so many branches of Christianity [eg Roman Catholic , Anglican , Methodist, Baptist . . . ]
that there is NO central Organisation.