The Barna Group's metrics have far less to do with being ‘post-Christian’ than with being ‘post-evangelical’.
The debate about the influence of what is termed culture on faith lies behind most conflict in the contemporary Western Church. Of course it all depends on how one defines “culture” and how one defines “faith.”
The Nigerian church is on the front lines of the encounter with militant Islam. Many Christians there are, at any moment, in danger of losing life or property. So they cannot afford flabby troops. Neither can we, actually, but we haven’t completely figured that out yet.