A Benedictine monk at Maria Laach, Odo Casel was a major force in the movement for renewal of liturgy in this century. He objected to rationalistic or anthropocentric interpretations of the Christian religion that would reduce it either to a collection of doctrines or to a moral code. Instead, he insisted that the core of Christian faith is the worship of a sacred mystery. For Casel, liturgy means "the making real of the mystery of Christ in the new alliance throughout the whole church, in all centuries; in it her healing and glory are made fact." This essay presents the heart of Casel's theology of mystery, summing up his view that the encounter with the divine Mystery in liturgy is "the most central and most essential action of the Christian religion."