On October 31, 1517, a 33-year-old Augustinian monk, parish priest, and professor of theology nailed "95 Theses" to the door of the Castle Church in Wittenberg, Germany, denouncing the papacy's sale of indulgences to finance the restoration of St. Peter's Basilica. (Indulgences are papal certificates offering remission of the temporal penalty due forgiven sin and granting sinners parole from the fires of purgatory.) That man, Martin Luther, launched the Protestant Reformation. And that date, October 31, has been commemorated long since as Reformation Day.

Lutherans have traditionally celebrated Reformation Day in quite triumphal fashion, with rousing renditions of Luther's hymn, "A Mighty Fortress," and with frequent anti-Catholic polemics as well. But last year, many Lutherans observed this day in a whole new way, by affirming with Roman Catholics a Joint Declaration on the Doctrine of Justification.

For centuries, the doctrine of the sinner's justification in the sight of God has been referred to as the "first and chief article" of Protestant Christianity, the article "on which the church stands or falls." Luther taught that believers are justified not by anything they do, but only because of Christ's righteousness, which God in his infinite mercy graciously reckons as their own. No one deserves this divine favor; we sinners deserve condemnation. But God has promised that those who trust in Christ alone for their salvation will not be judged on their own (de)merits, but on the basis of the genuinely meritorious life, death, and resurrection of their Savior, to whom they are united by the Holy Spirit and whose righteousness they wear as a sacred robe. Luther believed that the Holy Spirit indwells justified Christians, liberating ...