Another non-Lutheran discovers a Lutheran insight. Carl Trueman, professor at Westminster Seminary in Philadelphia, posts about theologians of the cross as opposed to theologians of glory. He explains it pretty well, I think:
One of the things that is so striking about the current revival of interest in Reformation theology, broadly conceived, is the absence of perhaps the most glorious contribution of Martin Luther to theological discourse: the notion of the theologian of the cross. . . .
At the heart of this new theology was the notion that God reveals himself under his opposite; or, to express this another way, God achieves his intended purposes by doing the exact opposite of that which humans might expect. The supreme example of this is the cross itself: God triumphs over sin and evil by allowing sin and evil to triumph (apparently) over him. His real strength is demonstrated through apparent weakness. This was the way a theologian of the cross thought about God.