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.