In his early work, The Heidelberg Disputation (1518), Luther distinguishes between two different sort of the theologians: a theologian of glory and a theologian of the cross. Regarding the theologian of glory, Luther writes:
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" (Rom. 1:20; cf. 1 Cor 1:21-25). This is apparent in the example of those who were "theologians" and still were called "fools" by the Apostle in Rom. 1:22. Furthermore, the invisible things of God are virtue, godliness, wisdom, justice, goodness, and so forth. The recognition of all these things does not make one worthy or wise.
In other words, the theologian of glory looks to God apart from his Word and speculates about him through the created order. There are two aspects to this. First, epistemically speaking, this knowledge is a sort of knowledge that comes from vision, rather than hearing. According to Aristotelian epistemology (which Luther was trained in and mentions derisively in passing) all human knowledge is described as a kind of intellectual vision. A person knows a thing because a sort of intellectual vision of a thing is imprinted on one's intellect. Logically speaking then, from the perspective of this epistemology, the more one would contemplate God through the visible creation, the more one would become like God in that God's own reality would imprint itself upon one.
Generally speaking, this is how many medieval theologians described the divine human-relationship as actualizing itself. For Thomas Aquinas, creation and revelation together stand as an analogy to the divine being. By the power of grace, they become transparent to the human intellect. The human subject is made up of the faculties of memory, intellect, and will. These correspond to Father, Son, and Holy Spirit. The created habitus of faith augments intellect, whereas memory is augmented by the habitus of hope and will by the habitus of love. These augmentations of the human faculty came about by an act of divine grace. As these habitus become virtues when exercised and feed by divine grace. They are divinely created qualities that make it possible for us to be pleasing to God and thereby finally participate in God's eternal act of self-knowledge. God is best described by Aquinas as a desiring subject. He knows himself and loves himself because of his superabundance of being. The more human become like him, the more pleasing they are and thereby can ascend to him. We can therefore see exactly what Luther means by the theology of glory. The human subject who knows God and thereby becomes like God in his majesty, can ascend to God because of his or her similarity and desirability to God. Luther condemns all of this.
Note that there is no suggestion here on Luther's part that this does not constitute (from a purely epistemic perspective) a valid knowledge of God ("Yet that wisdom is not of itself evil, nor is the law to be evaded; but without the theology of the cross man misuses the best in the worst manner"). Luther's point is that one who tries to interact with God through creation and law will ultimately enter into an unending project of self-deification and justification.  One will try to become like God to make a claim on God, as we can observe in the case of medieval theologians like Aquinas. This is, the essence of sin, namely the failure to be a receptive creature.
By contrast, the theologian of the cross holds to the flesh of Jesus where God hidden to vision in sufferings:
The manifest and visible things of God are placed in opposition to the invisible, namely, his human nature, weakness, foolishness. The Apostle in 1 Cor. 1:25 calls them the weakness and folly of God. Because men misused the knowledge of God through works, God wished again to be recognized in suffering, and to condemn "wisdom concerning invisible things" by means of "wisdom concerning visible things," so that those who did not honor God as manifested in his works should honor him as he is hidden in his suffering (absconditum in passionibus). As the Apostle says in 1 Cor. 1:21, "For since, in the wisdom of God, the world did not know God through wisdom, it pleased God through the folly of what we preach to save those who believe." 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. Thus God destroys the wisdom of the wise, as Isa. 45:15 says, "Truly, thou art a God who hidest thyself." So, also, in John 14:8, where Philip spoke according to the theology of glory: "Show us the Father." Christ forthwith set aside his flighty thought about seeing God elsewhere and led him to himself, saying, "Philip, he who has seen me has seen the Father" (John 14:9). For this reason true theology and recognition of God are in the crucified Christ, as it is also stated in John 10 (John 14:6) "No one comes to the Father, but by me." "I am the door" (John 10:9), and so forth.
Luther's point here is clear. Whereas the theologian of glory becomes like God through activity and vision, the theologian of the cross becomes a receptive creature by faith and hearing. There is nothing attractive about the crucified Jesus. In his weakness and condemnation, he does not seem like God, who as we know from the created order, is glorious, powerful and righteous. In the same way, the person of faith does not look attractive as the person who does works. They admit their sins and appear guilty. Nevertheless, they are receptive as true creatures should be. First, they see that their works are of no avail in light of cross.
Secondly, their knowledge of Christ comes by hearing and not by vision. God is not transparent in the cross (a mistake commonly made by modern proponents of the theology of cross), but is rather "hidden in his suffering" and therefore believe in on the basis of the Word. What this all appears to be a rejection of the Aristotelian concept of knowledge and virtue as being the basis of the divine-human relationship. The human being relies on the Word. The Word is not an analogy for a hidden invisible reality which thereby makes God's being transparent. Rather it tells us that all righteousness, glory, and power are hidden in Jesus and perceived only through hearing of faith. Later on in the Bondage of the Will (1525), Luther refined and expanded on this point by stating that God always acts under the form of his opposite: "“[t]hus when God makes alive he does it by killing, when he justifies he does it by making men guilty, when he exalts to heaven he does it by bringing down to hell.” The creature does not, as in the Aristotelian epistemology, have a partial intellectual vision of God made transparent by revelation. Rather, one has a Word and promise which stand in contradiction to which is visible, namely the condemnation of the law. This contradiction humbles reason and destroys our ability to use it or our moral agency to ascend into the hidden divine life. By this realize Luther comments that "I myself was offended more than once, and brought to the very depth and abyss of despair, so that I wished I had never been created a man, before I realized how salutary that despair was, and how near I was to grace.” For this reason faith and receptivity to the divine Word are the proper stance of the creature in relation to divine hiddenness: "“Thus God hides his eternal goodness and mercy under eternal wrath, his righteousness under iniquity. This is the highest degree of faith, to believe him merciful when he saves so few and damns so many, and to believe him righteous when by his own will he makes us necessarily damnable.”
In harmony with his instance that God only comes to us in a redeeming manner in the flesh of Christ and the reception of his tangible Word, is his condemnation of Enthusiasm. This particular emphasis that the Spirit only comes through the means of grace began in with his debates with Karlstadt and also Thomas Müntzer in the early 1520s. Much as Luther had insisted earlier on finding God hidden in the tangible flesh of the crucified Jesus, he now emphasized the need to not look for God in utterance of those who claimed the intangible Spirit but rather in the external means of grace. Later in the Smalkald Articles (1537) Luther would comment that he saw very little daylight between the Enthusiasts, the Pope and the Serpent in the garden of the Eden.  All claimed the ability to know God's hidden will and drew people away from external Word. In this, Luther posits that Enthusiasm and unbelief (as we will see further below) are the original sin. In a sense, the "Heavenly Prophets" as Luther referred to them were essentially of the same stripe as the theologians of glory that he had mentioned in 1518. Through the Spirit, without visible means, they claimed to have bridged the gap between themselves and the hidden divine will.
We may go a step further than Luther and observe that Enthusiasm is a logical development from the theologian of glory's attempt to bridge the gap between themselves and God through the law. In other words, as we observed earlier, the act of promising is always an act of self-donation. If I promise to do such-and-such, then I give myself over to you in order to fulfill the terms of that promise. Promising always involving giving of the self to the other in a tangible way. If we understand the underling logic of Luther's position in this way, the gospel goes hand-in-hand with a strong concept of the Incarnation (God's surrender of his very being to humans) and a tangible means of grace. God by making promises surrenders himself in the form an external sign to human beings. Receiving his self-donating promise through these means is to receive his tangible presence in, under, and with them.
By contrast, the law goes hand and hand with divine intangibility. This does not mean that God does not act through visible means as law, but it does means that God acts and present himself not "bound" to his gracious promises. As unbound by his promise of self-subjection and self-donation in the gospel, God is utterly threatening and unpredictable. In this situation, God is not my tangible object, rather I am his. Consequently, the Enthusiast who has chosen look for God in the sphere of the intangible, where God is not bound by his promise self-donation, must enter into a project of self-justification. He must now use the law a means of self-defense against the hidden God. This exactly mirrors the self-justification project of the theologian of glory. In order to bridge this gap between God and humans, the Enthusiast must set himself up as an alternative mediator. An alternative mediator is necessary to reveal the secret truth of God and thereby offering an opportunity of self-justification. Being able to make a claim over against the God effectively means that one is greater than that God. Therefore self-justification goes hand-in-hand with self-deification.