Dionysius the Areopagite exercised immense influence on medieval theology. This study considers various ways in which his doctrine of union with God in darkness marked the early Albert the Great and his student Thomas Aquinas. The Mystery of Union with God considers a broad range of themes in the early Albert's corpus and in Thomas that underlie their mystical theologies and may bear traces of Dionysian influence. These themes include the divine missions, anthropology, the virtues of faith and charity, primary and secondary causality, divine naming, and eschatology. The heart of this work offers detailed exegesis of key union passages in Albert's commentaries on Dionysius, Thomas's Commentary on the Divine Names, and the Summa Theologiae questions on Spirit's gifts of understanding and wisdom. The Mystery of Union with God offers the most extensive, systematic analysis to date of how Albert and Thomas interpreted and transformed the Dionysian Moses ""who knows God by unknowing."" It shows Albert's and Thomas's philosophical and theological motives to place limits on Dionysian apophatism and to reintegrate mediated knowledge into mystical knowing. The author surfaces many similarities in the two Dominicans' mystical doctrines and exegesis of Dionysius. This work prepares the way for a new consideration of Albert the Great as the father of Rhineland Mysticism. The original presentation of Aquinas's theology of the Spirit's seven gifts breaks new ground in theological scholarship. Finally, the entire book lays out a model for the study of mystical theology from a historical, philosophical and doctrinal perspective.