"Unum praedica, sapientiam crucis!" [There is one thing to preach, the wisdom of the cross!] That is the answer (in a sermon-fragment of 1515; WA 1, 52) which Luther gives to the vital question of the ministry [Predigtamt] of all ages: "What shall I preach?" The wisdom of the cross, the word of the cross, a great stumbling block to the world, is the proper content of Christian preaching, is the Gospel itself. So thinks Luther and the Lutheran Church with him. The Christian world regards that as a great onesidedness. The cross is just one part, among others, of the Christian message. The Second Article is not the whole Creed, and even in the Second Article the cross stands in the midst of other facts of salvation. What a narrowing of the Christian truth Luther is guilty of (so we are told by some Lutherans today) by his limiting real Christian theology to the theology of the cross. Is not there also a theology of incarnation and a theology of resurrection? Must not the theology of the Second Article be supplemented by a theology of the Third Article, a theology of the Holy Ghost and His activity in the church? Luther had, indeed, very much to say about these things also, e.g., in his doctrine on incarnation and in his theology of the sacraments. Besides, he had a more profound understanding of the article of creation than most theologians who preceded him.
Thus the question arises what that alleged narrowing, that much criticized onesidedness of Luther's theologia crucis, means. The theology of the cross obviously does not mean that for the theologian the whole church year shrinks to Good Friday. It rather means that one cannot understand Christmas, Easter, or Pentecost without Good Friday. Luther was, alongside of Irenaeus and Athanasius, one of the great theologians of the incarnation. He was that because he saw the cross behind the manger. He understood the victory of Easter as well as any theologian of the Eastern Church. But he understood it because he understood it as the victory of the Crucified. The same can be said of his understanding of the activity of the Holy Ghost. It is always the cross which illuminates all chapters of theology because the deepest nature of revelation is hidden in the cross. This being so, Luther's theologia crucis wants to be more than one of the many theological theories which have appeared in the course of the history of the Church. It claims to be, in contrast to another theology, which now prevails in Christendom and which Luther calls the theologia gloriae, the correct, the scriptural theology with which the Church of Christ stands and falls. Only of the preaching of this theology, Luther thinks, can it be said that it is the preaching of the Gospel.
Sasse, Letters to Lutheran Pastors 18