1 Corinthians 5:7-8
Christ our Pasch is immolated, alleluia: therefore let us feast with the unleavened bread of sincerity and truth, alleluia, alleluia, alleluia.
Haydock
Verse 6-8. Your glorying is not good, when you suffer such a scandal among you: you have little reason to boast of your masters, or even of the gifts and graces you received. A little leaven corrupteth the whole mass; a public scandal, when not punished, is of dangerous consequence. — Purge out the old leaven. He alludes to the precept given to the Jews of having no leaven in their houses during the seven days of the Paschal feast. For our Pasch, i.e. Paschal lamb, Christ is sacrificed: and Christians, says S. Chrys. must keep this feast continually, by always abstaining from the leaven of sin. Wi.
Denzinger
443: The Necessity of Preserving Theological Terminology and Tradition
GREGORY IX 1227-1241 From the letter "Ab Aegyptiis" to the theologians of Paris, July 7, 1228
Therefore, lest a rash and perverse dogma of this kind “as a canker spreads” [2 Tim. 2:17], and infects many and makes it necessary that “Rachel bewail her lost sons” [Jer. 31:15], we order and strictly command by the authority of those present that, entirely forsaking the poison mentioned above, without the leaven of worldly knowledge, that you teach theological purity, not “adulterating the word of God” [2 Cor. 2:17] by the creations of philosophers, lest around the altar of God you seem to wish to plant a grove contrary to the teaching of the Lord, and by a commingling of honey to cause the sacrifice of doctrine to ferment which is to be presented “with the unleavened bread of sincerity and truth” [1 Cor. 5:8]. But content with the terminology established by the Fathers, you should feed the minds of your listeners with the fruit of heavenly words, so that after the leaves of the words have been removed, “they may draw from the fountains of the Savior” [Is. 12:3 ]; the clear and limpid waters which tend principally to this, that they may build up faith or fashion morals, and refreshed by these they may be delighted with internal richness.