Christ is the historical real presence of God’s mercy in its definitive victory. As such, he is at once reality and sign: sacramentum and res sacramenti. The Church is the continuation, the abiding presence of Christ in the world. From Christ the Church has a sacramental structure: historical perceptibility in space and time, in which Christ, and the grace of Christ, are present. Thus the Church is the prime sacrament {Ursakrament). She cannot be an empty sign, since she is permanently united with Christ. The sacramental structure of the Church is set forth and perfected in the sacraments strictly so called: they are her essential actions, her full actuation. When the Church, as means of salvation, officially, socially, publicly, explicitly encounters the individual in the final actualization of her being, then we have sacraments in the strict sense. The notion of prime sacrament, therefore, is derived from Christology. It affords a principle for the understanding of the sacraments in general.
Karl Rahner, THE CHURCH AND THE SACRAMENTS, 1963