Catechism of the Catholic Church §329 — excerpt
St. Augustine says: "'Angel' is the name of their office, not of their nature. '" With their whole beings the angels are servants and messengers of God.
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: St. Augustine, En. in Ps. 103, 1, 15: PL 37, 1348