Matthew 16

The Pharisees were looking for a sign from God. They would not believe the word of God or of Jesus. They were demanding something that God was not going to do. He wanted them to believe the prophets and the signs that He had already given them - to use their faith. I believe He wants the same from us today, for us to trust Him as His word and spirit prompt. He talked about the condition of the heart as a yeast that spreads through bread. The yeast of Christ is the one that should spread, not the yeast of the one who will lead us into temptation. The disciples had a tendency to think and concentrate on the physical, and revert back to the human thought process of feeding their bodies. Sometimes we all do that.
8 Aware of this, Jesus said, "You of little faith! Why are you discussing among yourselves that you do not have bread? 9 Don't you understand yet? Don't you remember the five loaves for the 5,000 and how many baskets you collected? 10 Or the seven loaves for the 4,000 and how many large baskets you collected? (F) 11 Why is it you don't understand that when I told you, 'Beware of the yeast of the Pharisees and Sadducees,' it wasn't about bread?" 12 Then they understood that He did not tell them to beware of the yeast in bread, but of the teaching of the Pharisees and Sadducees.

Peter got a lot of things right, confessing Jesus as the Christ as He was willing to listen to the Spirit of God who had revealed it to him. It was something that was congruent with his spirit. Then in the next paragraph Peter fell far short as Jesus told them of His coming death.
22 Then Peter took Him aside and began to rebuke Him, "Oh no, Lord! This will never happen to You!" 23 But He turned and told Peter, "Get behind Me, Satan! You are an offense to Me because you're not thinking about God's concerns, but man's."
Many times it's hard to take up our cross and follow Jesus when it's not where we want to go or what is congruent with our idea of the journey. 26 "What will it benefit a man if he gains the whole world yet loses his life? Or what will a man give in exchange for his life? 27 For the Son of Man is going to come with His angels in the glory of His Father, and then He will reward each according to what he has done. 28 I assure you: There are some standing here who will not taste death until they see the Son of Man coming in His kingdom."