The Missionary Character of the Liturgy
While the liturgy daily builds up those who are within into a holy temple of the Lord, into a dwelling place for God in the Spirit (Eph. 2:21-22), to the mature measure of the fullness of Christ (Eph. 4:13), at the same time it marvelously strengthens their power to preach Christ, and thus shows forth the Church to those who are outside as a sign lifted up among the nations
(Is. 11:12) under which the scattered children of God may be gathered together (John 11:52), until there is one sheepfold and one shepherd (John 10:16).
-Constitution on the Sacred Liturgy, Sacrosanctum Concilium # 2