Chapter 18 — Part II: The Sacraments — The Holy Eucharist
Section 34. Unleavened Bread Not Essential — This quality of the bread, however, is not to be deemed so essential that, if it be wanting, the Sacrament cannot exist; for both kinds are called by the one name and have the true and proper nature of bread. No one, however, is at liberty on his own private authority, or rather presumption, to transgress the laudable rite of his Church. And such departure is the less warrantable in priests of the Latin Church, expressly obliged as they are by the supreme Pontiffs, to consecrate the sacred mysteries with unleavened bread only.