5 And when his disciples were come to the other side, they had forgotten to take bread.
6 Then Jesus said unto them, Take heed and beware of the leaven of the Pharisees and of the Sadducees.
7 And they reasoned among themselves, saying, It is because we have taken no bread.
8 Which when Jesus perceived, he said unto them, O ye of little faith, why reason ye among yourselves, because ye have brought no bread?
9 Do ye not yet understand, neither remember the five loaves of the five thousand, and how many baskets ye took up?
10 Neither the seven loaves of the four thousand, and how many baskets ye took up?
11 How is it that ye do not understand that I spake it not to you concerning bread, that ye should beware of the leaven of the Pharisees and of the Sadducees?
Jesus was addressing the leaven of the Pharisees but He was also addressing the fact that the disciples couldn't understand who Jesus was and remember the miracles He did with the loaves of bread.
Jesus is saying don't you remember...
the plagues I sent upon Egypt, the deliverance from Egypt, the parting of the Red Sea, the manna from heaven, your shoes never wore out, the water that gushed from the Rock, the battles you won, Abraham's descendant's more than the grains of sand, David slaying Goliath, Esther saving the Jews, Nehemiah building the wall of Jerusalem in 52 days, all the miracles I did and raising people from the dead, the sun stood still and the moon stayed for Joshua in battle, I walked on water, calmed the roaring sea, gave you prophets to warn you, sent John the Baptist to bring repentance, gave you the beginning and the end of all things. I gave you My Holy Spirit.
Don't you understand who I AM.
John 8:58 Jesus said unto them, Verily, verily, I say unto you, Before Abraham was, I am.
Exodus 3:14 And God said unto Moses, I Am That I Am: and he said, Thus shalt thou say unto the children of Israel, I Am hath sent me unto you.
Do we remember who Jesus is when life gets difficult, when there are no answers to be found, when fear, worry and anxiety overtakes us, when mountains are too big to climb, when waters get rough, when battles are overwhelming, when our bodies get weak, when life doesn't seem fair, when injustice has not seen justice...do we understand who Jesus is? Do we remember the loaves and the cross, the BREAD OF LIFE?
Doubting who Jesus is, is like forgetting all His wondrous miracles of love that He has poured on us. Leaven represents sin, it is pride.
Proverbs 8:13 The fear of the Lord is to hate evil: pride, and arrogancy, and the evil way, and the froward mouth, do I hate.
Pride is found in the outer layer of our hearts and unbelief is found in the inner layer. Unbelief leads us to not understand nor remember who Jesus is or what He has done. Where there is doubt, there is leaven.