Dietrich Bonhoeffer:

Anybody who lives beneath the Cross and who has discerned in the Cross of Jesus the utter wickedness of all men and of his own heart will find there is no sin that can ever be alien to him. Anybody who has once been horrified by the dreadfulness of his own sin that nailed Jesus to the Cross will no longer be horrified by even the rankest sins of a brother. Looking at the Cross of Jesus, he knows the human heart. He knows how utterly lost it is in sin and weakness, how it goes astray in the ways of sin, and he also knows that it is accepted in grace and mercy. Only the brother under the Cross can hear a confession.

It is not experience of life but experience of the Cross that makes one a worthy hearer of confessions. 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 also does not know that man is destroyed only by his sin and can be healed only by forgiveness. Only the Christian knows this. In the presence of a psychiatrist I can only be a sick man; in the presence of a Christian brother I can dare to be a sinner. The psychiatrist must first search my heart and yet he never plumbs its ultimate depth. The Christian brother knows when I come to him: here is a sinner like myself, a godless man who wants to confess and yearns for God’s forgiveness. The psychiatrist views me as if there were no God. The brother views me as I am before the judging and merciful God in the Cross of Jesus Christ.

Life Together (published in German in 1939; English edition: Harper and Row, 1954), p. 118-119.