Lauda Sion Salvatorem
Lauda Sion Salvatorem (Zion Lift Thy Voice and Sing) is a sequence for the Roman Catholic Mass of Corpus Christi. It was written by St. Thomas Aquinas in or around the year 1264 at the request of Pope Urban IV, when the Feast of Corpus Christi was new. 
Despite being only a hymn, it explains much theology, recalling the institution of the Eucharist, the foreshadowing of the Eucharist in the Old Testament, and the effects of the Eucharist on the faithful and the wicked. It mentions even subtleties of the doctrine of Transubstantiation.
- See [http://www.newadvent.org/cathen/09036b.htm Catholic Encyclopedia