Yes, Obama's spiel at the Charleston church was spectacular, artful, wonderful. God's grace is great — if you qualify and can get it. But here's my problem: after all that forgiveness and grace, how many seriously committed Christians — including black Christians — are going to allow for gay marriage? And, further, stand up to say so?
And no, not that you asked me, but the Constitution is not an intrinsically Christian document; its origins, on the contrary, are in the Enlightenment, a momentously secular unfolding.
And no, the United States is not by definition a Christian commonwealth, whatever that might mean. It's a richer, more complicated, polyglot thing. I say that as a Jew — and as an American.
All this is so obvious I shouldn't even have to say it. But some of the avowedly Christian reaction to gay marriage occasions it.