Catechism of the Catholic Church §813 — excerpt
The Church is one because of her source: "the highest exemplar and source of this mystery is the unity, in the Trinity of Persons, of one God, the Father and the Son in the Holy Spirit."
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: UR 2 # 5 · GS 78 # 3 · UR 2 # 2 · St. Clement of Alexandria, Paed. 1, 6, 42: PG 8,300