The most experienced psychologist or observer of human nature knows infinitely less of the human heart than the simplest Christian who lives beneath the Cross of Jesus.
The greatest psychological insight, ability, and experience cannot grasp this one thing: what sin is.
Worldly wisdom knows what distress and weakness and failure are, but it does not know the godlessness of man. And so it does not know that man is destroyed only by his sin and can be healed only by forgiveness. Only the Christian knows this.
-Bonhoeffer. Click on the Theologica link for the extended quote and the reference.