This study seeks to define “mysticism” historically by looking back to the concept mystical theology (μυστική θεολογία / mystica theologia) and the association of books under this category. The first Latin canon of mystical theology developed from the ninth to the middle of the thirteenth century around translation and commentary on the writings of Dionysius by John Eriugena, Thomas Gallus, Robert Grosseteste, and Albert the Great. The second Latin canon of mystical theology developed from the thirteenth to the end of the fifteenth century and centered upon the more practical task of teaching an advanced form of unitive and apophatic prayer (largely by Bonaventure, Hugh of Balma, and Jean Gerson). Nonetheless, the writings of Dionysius and the method of apophaticism—whether understood as negative predication (apophatic theology) or imageless (apophatic) prayer—remained fundamental. The criterion of immediacy also played a central role in the development of the concept, especially after the thirteenth century. Martin Luther inherited this concept of mystical theology. Reading Dionysius through the comments of Gerson and Johann Eck, Luther understood mystical theology as an advanced form of imageless prayer that sought an unmediated, experiential knowledge of God in majesty (“the naked God”) through union. Luther’s mature writings rejected any attempt to know God in this direct way. However, drawing on Augustine and Johann Tauler, Luther redirected the elements of experiential knowledge and the method of negation that he learned in mystical literature. Rather than addressing the disproportion between the finite and the infinite, Luther’s version of negative theology addressed the disproportion between the word of the Law and the word of the Gospel. In turn, Luther redirected Dionysius’s original emphasis on the mediated nature of all theology towards the pastoral task of assurance. For Dionysius direct knowledge of God was metaphysically impossible. For Luther direct knowledge of God was not impossible, but disastrous. Only in the negation of the Law and “darkness of faith” in the spoken, external words of the Gospel could assurance be found.