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.