“In his struggles with penance and confession, he [Martin Luther the monk-scholar] wrestled with Psalm 19:12, ‘Clear thou me from hidden faults’ (ASV). Luther’s problem was never whether his sins were large ones or small ones, but whether in fact he had confessed every single one. What about the sins he could not remember? What about the sins committed in his sleep? Luther anticipated Freud by recognizing a depth-dimension to the human person and by refusing to limit the effects of sin to the conscious mind alone. Such a radical reading of the human situation could only be answered with an even more radical reading of divine grace. …
Luther’s new insight was that the imputation of Christ’s alien righteousness was based, not on the gradual curing of sin, but rather on the complete victory of Christ on a cross. The once-for-allness of justification was emphasized: ‘If you believe, then you have it!’ Nor is there any direct correlation between the state of justification and one’s outward works, as Luther made clear in his sermon on the pharisee and the publican (1521): ‘And the Publican fulfills all the commandments of God on the spot. He was then and there made holy by grace alone. Who could have foreseen that, under this dirty fellow?’
Luther’s doctrine of justification by faith fell like a bombshell on the theological landscape of medieval Catholicism. It shattered the entire theology of merit and indeed the sacramental-penitential basis of the church itself.”
– Timothy George essay on Luther in Reading Romans Through the Centuries: From the Early Church to Karl Barth (Brazos Press: 2005) pp. 115-116.