The theology of the cross, or theologia crucis, is a term coined by the German theologian Martin Luther to refer to the belief that the cross is the only source of spiritual knowledge concerning who God is and how God saves. Only at the cross does a fallen human being gain the understanding that is the result of the indwelling of the Holy Spirit at conversion (1 Corinthians 12:13; Romans 8:9; Ephesians 1:13–14). Cross theology is contrasted with the theology of glory, or theologia gloriae, which places greater emphasis on human abilities and human reason. Luther first used the term theologia crucis in the Heidelberg Disputation of 1518, where he defended the Reformation doctrines of the depravity of man and the bondage of the will to sin.
The primary difference between the theology of the cross and the theology of glory is the ability or inability of man to justify himself before a holy God. The theologian of the cross sees as inviolate the biblical truths of man’s inability to earn righteousness, the inability for humans to add to or increase the righteousness attained by Jesus’ sacrifice on the cross, and the only source of man’s righteousness coming from outside of ourselves. The cross theologian agrees wholeheartedly with the Apostle Paul’s assessment of the human condition: “I know that nothing good lives in me, that is, in my sinful nature” (Romans 7:18). The cross theologian rejects the idea that man can attain righteousness in any degree by keeping the works of the law, but is saved and sanctified solely by grace (Romans 3:20; Ephesians 2:8–9).
Theologians of glory, on the other hand, see good in humans and ascribe to them the ability to do the good that lies within them. They believe that there remains, after the fall, some ability to prefer good over evil and to choose the good. Most significantly, glory theology posits that humans cannot be saved without participating in or cooperating with the righteousness given by God. This is the classic works vs. faith debate which has long been fueled by a misunderstanding of certain passages in the book of James. James 2:17–18 is interpreted to mean that we are justified by our works, while James is actually saying that those who have been justified by faith in the work of Christ on the cross will produce good works as evidence of true conversion, not that conversion is obtained by good works.
It should be noted that theology of the cross is not the sentimental idea that Jesus is made more attractive to us by His identifying with our trials and tribulations. While Jesus certainly does identify with our suffering, our suffering is not somehow made nobler because of it. Our suffering is the byproduct of the fall of mankind into sin, whereas Jesus’ suffering was that of an innocent Lamb slaughtered for the sake of others’ sin, not His own. Nor is cross theology our identification with His suffering through our own, which pales in comparison to what He went through. In the end, Jesus suffered and died because nobody identified with Him. The people cried, “Crucify him!” One of His disciples betrayed Him, another denied Him three times, and the rest abandoned Him and fled. He died alone, forsaken even by God. So to attempt to unite ourselves with Him in His suffering is to diminish His sacrifice and exalt our own sufferings to a level never intended by the theology of the cross which Luther posited.
 Got Questions Ministries. (2010). Got Questions? Bible Questions Answered. Bellingham, WA: Logos Bible Software.