“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.
— Hermann Sasse, Theologia Crucis