Of the kind of Bread and Wine that ought to be used for the Consecration of this Sacrament

THOSE conditions must be observed which are essential for bread and wine to be. That alone is called wine, which is liquor pressed out of grapes:* nor is that properly called bread, which is not made of grains of wheat. Substitutes for wheaten bread have come into use, and have got the name of bread; and similarly other liquors have come into use as wine: but of no such bread other than bread properly so called, or wine other than what is properly called wine, could this Sacrament possibly be consecrated: nor again if the bread and wine were so adulterated with foreign matter as that the species should disappear. A valid Sacrament may be consecrated irrespectively of varieties of bread and wine, when the varieties are accidental, not essential. The alternative of leavened or unleavened bread is an instance of such accidental variety; and therefore different churches have different uses in this respect; and either use may be accommodated to the signification of the Sacrament. Thus as Gregory says in the Register of his Letters*: "The Roman Church offers unleavened bread, because the Lord took flesh without intercourse of the sexes: but other Churches offer leavened bread, because the Word of the Father, clothed in flesh, is at once true God and true man." Still the use of unleavened bread is the more congruous, as better representing the purity of Christ's mystical Body, the Church, which is figured in a secondary way (configuratur) in this Sacrament, as the text has it: Christ our passover is sacrificed: therefore let us feast in the unleavened bread of sincerity and truth (1 Cor. v, 7, 8).

This shuts out the error of some heretics who say that this Sacrament cannot be celebrated in unleavened bread: a position plainly upset by the authority of the gospel, where we read (Matt. xxvi, 17: Mark xiv, 12: cf. Luke xxii, 7) that the Lord ate the passover with His disciples, and instituted this Sacrament, on the first day of the azymes, at which time it was unlawful for leavened bread to be found in the houses of the Jews (Exod. xii, 15); and the Lord, so long as He was in the world, observed the law. It is foolish then to blame in the use of the Latin Church an observance which the Lord Himself adhered to in the very institution of this Sacrament.*

4.68 : The Explanation of a Text
4.70 : That it is possible for a man to sin after receiving Sacramental Grace