Scattered throughout his many writings and lectures Karl Rahner frequently, directly and indirectly, addresses the topics of ecumenism and ecumenical theology. The purpose of this article is to demonstrate the connection between Rahner’s mystical transcendental theology and his ecumenical theology. This is done in two successive steps. Firstly, the contours of Rahner’s mystical transcendental theology are developed by tracing Rahner’s foundational thought from Spirit in the World to Hearers of the Word and, finally, to his doctrine of the supernatural existential. The conversation then moves to the foundations of Rahner’s ecumenical thought as it is articulated in his article Some Problems in Contemporary Ecumenism.