Paul says in 1 Corinthians 13:13, "The greatest of these is love."...Paul, in this passage, properly speaks about the love toward one's neighbor and indicates that love is the greatest, because it has the most fruit. Faith and hope have to do only with God. But love has infinite offices outwardly toward humanity. Indeed, let us grant to the adversaries that love toward God and our neighbor is the greatest virtue, because the chief commandment is this: "You shall love the Lord your God" (Matthew 22:37). But how will they conclude from this that love justifes? They say, "The greatest virtue justifies." By no means! For just as the greatest or first, Law does not justify, so also the Law's greatest virtue does not justify. But the virtue that justifies recieves Christ, which brings to us Christ's merits, by which we recieve grace and peace from God. This virtue is faith. As it has often been said, faith is not just knowledge. But it is willing to receive or take hold of those things that are offered in the promise about Christ. Furthermore, this obedience toward God (i.e., to want to receive the offered promise) is no less a divine service (latreia) than is love. God wants us to believe Him and to receive from Him blessings. He delcares this to be true divine service.
Apology of the Augsburg Confession V (III) 104-107 (225-228).