The Cross Alone is our Theology
"But far be it from me to boast except in the cross of our Lord Jesus Christ, by which the world has been crucified to me, and I to the world." - St. Paul's letter to the Galatians 6:14
Martin Luther once stated that God can only be found in the incarnate, humiliated, and crucified Jesus Christ. In his 1518 Heidelberg Disputation, Luther argued that God’s greatest work can only be seen through suffering and the cross. Thus we preach Christ and Him crucified. This is what is known as Luther’s theology of the cross. So the cross alone is our theology [Crux Sola Nostra Theologia.]
We look at the cross of Jesus and His suffering and death to understand God, for behind the cross lies God’s glory and power, hidden to all but believers. Such a theology maintains that without the cross of Jesus Christ, God cannot truly be found. Luther once quipped, “Apart from the cross, one only finds a false god.”
The theology of the cross, then, maintains that through the suffering and death of Jesus, God revealed Himself; for at the cross both God’s love and His mercy are displayed and at work for the world lost in darkness and sin.
Although God’s work of salvation is hidden in the cross of Christ, still, He is present and active in that work. Just so, through suffering in the life of the believer, God is both present and actively at work to do His will and for His glory.
 Luther Works: American Edition. 31:53
 Schmidt, Alvin J. Hallmarks of Lutheran Identity, 208
The Spirituality of the Cross, by Gene Edward Veith, Jr. See especially chapter 3, “The Theology of the Cross: The Hiddenness of God." [Kindle Edition]
The Genius of Luther’s Theology, by Robert Kolb and Charles P. Arand. See especially the discussion in chapter 4, pp. 80 and following. [Kindle Edition]
Hallmarks of Lutheran Identity, by Alvin J. Schmidt. See chapter 15, “Theology of the Cross: Luther’s Unique Insight. [Kindle Edition]
On Being a Theologian Of The Cross, by Gerhard O. Forde. See especially his Introductory Matters: “Two Stories: The Glory Story and the Cross Story,” and “Two Ways of Being a Theologian.” [Kindle Edition]