Justification for Christ’s sake, sola gratia et sola fide, is for Lutherans as for St. Paul, the article by which the church stands or falls. it is the centerpiece, which holds all the other articles of the Gospel together. And it more than anything else defines the Lutheran confession vis a vis Rome.
But the Wittenberg Reformation cannot be defined by reference only to the Roman Catholic alternative. The demarcation line against Geneva is just as constitutive for the Lutheran confession. And there everything comes to a head in the mystery of the true presence of the Lord’s body and blood in the Sacrament of the Altar. This Calvin denied as much as Zwingli. It is a profound untruth of modern “ecumenical” propaganda, that the difference between Calvinism and Lutheranism is purely over the “mode” of Christ’s sacramental presence, not over the fact. No, it is that fact which the Lutheran confession affirms, and the Calvinist denies, as the Formula of Concord amply demonstrates in its seventh article…
In sum, the two constitutive foci for the Church of the Augsburg Confession are justification and the Holy Supper. These stand at the center of her confession, and without them, she ceases to be.
We have already noted the deep links between justification and the Resurrection. There is a similar link between the Resurrection and the Supper. Oscar Cullman has pointed out that the apostolic church understood the Sacrament as a continuation of the Lord’s Resurrection appearances [Oscar Cullman, Early Christian Worship, tr. A. Todd and J. Torrance (Chicago: Regnery, 1953), pp. 14-20)], but now invisibly. Christianity’s Founding Fact is celebrated every “first” or “Lord’s Day” in the Lord’s Supper. As the Apology puts it (X, 4): “We are talking about the presence of the living Christ, knowing that ‘death no longer has dominion over him.'”
-Kurt Marquart, “The Church in the Twenty-First Century:Will There Be a Lutheran One?” in All Theology Is Christology.