What then is faith? Faith is constant assent to God’s every word, and it does not exist outside of God’s Spirit renewing and enlightening our hearts. Now the Word of God is both Law and Gospel. Threats have been added to the Law. Scripture calls it fear when we believe these threats, and faith when we trust in the Gospel or God’s promises. Fear does not justify without faith. Otherwise the despairing and damned would be justified. But those who fear in this way do not glorify God or believe his every word, since they do not believe in his promises. Therefore faith alone justifies.
From Loci Communes by Phillip Melanchthon, translated by Christian Preus.