When Christians talk about the theology of the cross, they contrast it with the theology of glory. What's the difference between the two? Here's a brief explanation. It's taken from my booklet, Why Lutherans Sing What They Sing.
In Theses 19 and 20 of the Heidelberg Disputation, Martin Luther separates the wheat from the chaff, the true theologians from those in the ranks of the wannabes.
Thesis 19: That person does not deserve to be called a theologian who looks upon the invisible things of God as though they were clearly perceptible in those things which have actually happened [or, “have been made” quae facta sunt].
Thesis 20: He deserves to be called a theologian, however, who comprehends the visible and manifest things of God seen through suffering and the cross. (Luther’s Works, American Edition, ed. Helmut T. Lehmann, vol. 31, ed. Harold J. Grimm [Philadelphia: Fortress Press, 1971], p. 40)
In other words, no man deserves to be called a theologian unless the entire corpus of his theology is crucified. The sham-theologian, Luther says, fools himself into thinking that he can perceive who God really is in those things which are accessible to human experience, investigation, and reason. He presumes to recognize the invisible things of God (i.e., His “virtue, godliness, wisdom, justice, goodness, and so forth”) in the visible things of creation, but “the recognition of all these things does not make one worthy or wise” (AE, vol. 31, p. 52). “Now it is not sufficient for anyone, and it does him no good to recognize God in his glory and majesty, unless he recognizes him in the humility and shame of the cross” (AE, vol. 31, pp. 52-53). The uncrucified god is a false god for the true God cannot be known, cannot be recognized, cannot be confessed until and unless He is comprehended in the crucified Man, Jesus of Nazareth. Because “true theology and recognition of God are in the crucified Christ,” the crucifix is not only the ultimate but the ongoing epiphany wherein God reveals how He comes to His people and brings His people into Himself (AE, vol. 31, p. 53).
All theology must therefore be crucified. For instance, God the Father, Maker of heaven and earth, is not known as God the Father in the created things of heaven and earth by themselves. Visible creation certainly testifies that there is a Maker (Romans 1:20), but that God remains nameless and unknown as God our Father until He is known in His incarnate and crucified Son. The theology of creation must therefore be crucified for the God of creation truly to be known. Similarly, the Holy Spirit is unknown and unknowable except in the crucified Son, for the Spirit “bears witness of” and “glorifies” Christ (St. John 15:26; 16:14). The theology of the Spirit must therefore be crucified for the Holy Spirit truly to be known.
Who God is and how He deals with us is made known “through suffering and the cross,” as Luther summarily says. In other words, God is who and God is where man by nature supposes He is not. Luther was fond of quoting the prophet Isaiah in this regard: “Truly, Thou art a God who hidest Thyself” (Isaiah 45:15). God camouflages Himself beneath His seeming opposite: His glory is hidden beneath the inglorious cross, His strength hidden in weakness, wisdom in folly, exaltation in humiliation. Yet, this divine concealment is simultaneously divine revelation: His glory is made known precisely in the cross, His strength in weakness, His wisdom in folly, His exaltation in humiliation. These are revealed, however, solely to those have “seeing ears,” who behold what their ears are told in the Word of Christ. Only those who heed the Word of Christ see through these masks of God, that is, only they see God behind His seeming opposite, His outward disguise. Only those who know God in the crucified Christ know the God who hides Himself, and so only they will seek and find Him where His Word has promised He is and will be. On the other hand, those who heed not the Word of Christ, but their own natural experience, investigation, and reason will search for God and even possibly think that they have found Him. But, alas, they will be gravely disappointed. For all those who think they have laid hold of God where God is not, have really laid hold of an idol, an idol which is the mask and jaws of the devil himself (1 Corinthians 10:19-20).
The God who hides and reveals Himself in His crucified Son also hides and reveals Himself in the ways and means whereby this crucified Son comes to us. Everything by which God imparts Himself to us and brings us into Himself must bear the cruciform image of the Christ. Therefore, in virtually the same breath St. Paul calls the cross and the preaching of this cross “foolishness” to the world (1 Corinthians 1:18, 21). Just as the sin-blinded world cannot see God in the crucified Christ, so the world cannot see God in the means whereby the crucified Christ comes to us: in preaching. And so it is with every other way and means by which the hidden God comes to us. The God who is hidden in the “foolishness” of the cross is hidden in the “foolishness” of Baptism’s water, the Eucharist’s bread and wine, the Absolution’s human voice and touch. The offense of the cross now rests within the pulpit, upon the altar, in the font, at the confessional chair. Everything that belongs to God must be crucified, that is, it must hide God so that only those who heed His Word will find Him there, revealing and giving Himself.