The Blindness of Human Reason
The blindness of human reason is so incomprehensible and infinite that it cannot form sound judgments even about life and works, much less about the doctrine of faith. Therefore we must battle unremittingly not only against the opinions of our own heart, on which by nature we would rather depend in the matter of salvation than on the Word of God, but also against the false front and saintly appearance of self-chosen works.
From the “Lectures on Galatians” (1535) by Martin Luther (Vol. 27, p. 57, Luther's Works, Concordia Publishing House, St. Louis, 1964).