The author assembles and interprets archival materials, advancing a preliminary assessment of the significance of mysticism for the development of Lonergan's intellectual project. Lonergan assigns priority to a mysticism of transforming union as the existential principle from which flow charitable service and theological reflection. The density of Lonergan's published account of religious experience is relieved by an exploration of its relation to the thought of Karl Rahner, Harvey Egan, and William Johnston. A suggestion is made about the continued importance of systematic reflection for the refined articulation of religious experience.