Catechism of the Catholic Church §864 — excerpt
"Christ, sent by the Father, is the source of the Church's whole apostolate"; thus the fruitfulness of apostolate for ordained ministers as well as for lay people clearly depends on their vital union with Christ.
This translation is under copyright, so only the cited words are quoted here. Follow the link for the full text. Translation: English translation of the Catechism of the Catholic Church, 2nd ed. (Libreria Editrice Vaticana), as posted at vatican.va.
Also cites: AA 4 · AA 3