Matthew 16:8

Thus He takes this occasion to instruct them what is meant by the five loaves and the seven loaves, the five thousand and the four thousand, who were fed in the desert. For if the leaven of the Pharisees and Sadducees signified not earthly food, but corrupt traditions and heretical dogmas, why should not the food with which the people of God is nourished signify the true and uncorrupt doctrine?

Wherefore also He works another miracle, and then and not till then He reproves; I mean, He brings forward what they were reasoning in their hearts. But what were their reasonings? "Because," so it is said, "we have taken no bread." For as yet they were full of trepidation about the purifications of the Jews, and the observances of meats. Wherefore on all accounts He attacks them even with severity, saying, "Why reason ye in yourselves, O ye of little faith, because ye have brought no bread? Perceive ye not yet, neither understand? Have ye your heart hardened? Having eyes, see ye not? Having ears, hear ye not? Do ye not remember the five loaves of the five thousand, and how many baskets ye took up? neither the seven loaves of the four thousand, and how many baskets ye took up?" Seest thou intense displeasure? For nowhere else doth He appear to have so rebuked them. Wherefore then doth He so? In order again to cast out their prejudice about the meats. I mean that with this view, whereas then He had only said, "Perceive ye not, neither understand?" in this place, and with a strong rebuke, He saith, "O ye of little faith." For not everywhere is lenity a good thing. And as He used to allow them freedom of speech, so doth He also reprove, by this variety providing for their salvation. And mark thou at once His reproof, how strong, and His mildness. For all but excusing Himself to them for His severe reproofs to them, He saith, "Do ye not yet consider the five loaves, and how many baskets ye took up; and the seven loaves, and how many baskets ye took up?" And to this end He sets down also the numbers, as well of the persons fed as of the fragments, at once both bringing them to recollection of the past, and making them more attentive to the future.

(Verses 8 and following.) But Jesus, being aware of this, said to them: Why are you discussing among yourselves, O you of little faith, that you have no bread? Do you still not understand or remember the five loaves for the five thousand men, and how many baskets you took up? Nor the seven loaves for the four thousand men, and how many baskets you took up? How is it that you do not understand that I did not speak to you about bread? Beware of the leaven of the Pharisees and Sadducees. Then they understood that he did not tell them to beware of the leaven of bread, but of the teaching of the Pharisees and Sadducees. By occasion of the precept, which the Savior had commanded, saying: Beware of the leaven of the Pharisees and Sadducees, He teaches them what the five loaves and the seven signify: the five thousand men, and the four thousand, who were fed in the wilderness. Although the greatness of the miracles is clear, yet something else is shown in spiritual understanding. For if the leaven of the Pharisees and Sadducees signifies not bodily bread, but perverse traditions and heretical doctrines, why do the foods, with which the people of God are nourished, not signify the true and complete teaching? Let someone ask and say: How is it that they did not have bread, who, when seven baskets were filled immediately, they got into the boat and came to the region of Magadan; and there the sailors hear that they should beware of the leaven of the Pharisees and Sadducees? But the Scripture testifies that they forgot to take bread with them. This is the leaven that the Apostle speaks of; A little leaven leavens the whole lump (I Cor. V, 6). Marcion and Valentinus and all heretics had this kind of leaven, which is to be avoided in every way. This fermentation has the power that if it is mixed with flour, which seemed small, it grows into something greater, and attracts the entire mixture to its flavor ((or conversion)) : and so also heretical doctrine, if it has thrown even a small spark into your heart, in a short time it grows into a great flame, and draws the entire possession of the person to itself. Finally, it follows: Then they understood that he did not say to beware of leavened bread, but of the doctrine of the Pharisees and Sadducees.