Catechism of the Catholic Church §233 — excerpt
Christians are baptized in the name of the Father and of the Son and of the Holy Spirit: not in their names, for there is only one God, the almighty Father, his only Son and the Holy Spirit: the Most Holy Trinity.
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: Profession of faith of Pope Vigilius I (552): DS 415
