In modern American culture we tacitly agree on a division of our lives into three spheres: Private; Public; and National. Private is what you do behind closed doors. Public is what is socially acceptable. And National is the realm Reinhold Niebuhr was addressing in Moral Man and Immoral Society: the one where we have to be "selfish" because the concern is for the survival of the group (the "nation"). Public life, of course, is also concerned with the behavior of the group, so often, in these more or less ecumenical times, we try to relegate religion to the Private sphere, lest the Public become a forum for squabbling over competing visions of the proper role of the Public.
This is where McKibben's argument goes, however: if we are Privately Christian, what difference does that make? Is Christianity really all about you? If we are, indeed, privately Christian, here's the paradox: what really distinquishes us from non-Christians, except a confession of faith, a matter of where we spend an hour on Sunday, or just a general attituude that ours is a "Christian nation."
But if we are to be publicly Christian, how do we behave? What would the role of the church be then? Of worship? Is McKibben right when he says:
Plenty of vital congregations are doing great good works - they're the ones that have nurtured me - but they aren't where the challenge will arise; they've grown shy about talking about Jesus, more comfortable with the language of sociology and politics"
I can tell you that my first exposure to the language of politics was in seminary. And my church, the UCC, seems more comfortable talking about politics, than talking about God (ask me about it in comments, I'll give you my anecdote on that one).
Is this paradox a true paradox? Or one of our making? And how do we unmake it?