Chapter 18 — Part II: The Sacraments — The Holy Eucharist
Section 31. The Sacramental Bread Should Be Unleavened — But as wheaten bread alone is to be considered the proper matter for this Sacrament -- a doctrine which has been handed down by Apostolic tradition and confirmed by the authority of the Catholic Church -- so it may be easily inferred from the doings of Christ the Lord that this bread should be unleavened. It was consecrated and instituted by Him on the first day of unleavened bread, on which it was not lawful for the Jews to have anything leavened in their house.