14 Now the disciples had forgotten to take bread, neither had they in the ship with them more than one loaf.
15 And he charged them, saying, Take heed, beware of the leaven of the Pharisees, and of the leaven of Herod.
16 And they reasoned among themselves, saying, It is because we have no bread.
17 And when Jesus knew it, he saith unto them, Why reason ye, because ye have no bread? perceive ye not yet, neither understand? have ye your heart yet hardened?
18 Having eyes, see ye not? and having ears, hear ye not? and do ye not remember?
19 When I brake the five loaves among five thousand, how many baskets full of fragments took ye up? They say unto him, Twelve.
20 And when the seven among four thousand, how many baskets full of fragments took ye up? And they said, Seven.
21 And he said unto them, How is it that ye do not understand?
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Jesus' disciples were mostly simple people, and therefore they understood His words so literally. They hear about leaven and immediately decide that He is speaking about a shortage of food. It appears to them that the Lord is reproaching them, that He is displeased because they did not calculate the food for themselves and the people. But Christ's words were figurative, offering an image of the Pharisaism that could begin to grow in them. This is what He was warning them against.
The Savior's words are sharp enough. He really is reproaching them, but not because there is no bread. He reproaches them because they do not remember the miracle of multiplication that happened so recently. It turns out that they are so concentrated on themselves, on the things for which they blame themselves, that they do not pay attention to what is happening around them and forget the miracle in an instant.