The scriptures read are Amos 8, Psalm 114 and Matthew 16.
Amos 8. As fruit ripens, so does sin. God is fed up with Israel and specifically its mistreatment of the poor. Right now people can't wait for holy days to be over so they can get back to the business of exploiting the needy. One day they will search high and low for God's word in vain.
Psalm 114. A powerful choral version of this psalm. If you have the time, here is Mendelssohn's version with Psalm 115 thrown in as a bonus.
Matthew 16. Jesus' opponents want a miraculous sign to validate him as a prophet. Jesus promises them the sign of Jonah, swallowed by a fish and regurgitated onto land 3 days later. This is an enigmatic reference to Jesus' upcoming death, entombment and resurrection.
The yeast of the Pharisees and Sadducees is their influence. One group was popular with the people, the other had political power: a deadly combination, especially as enemies of Jesus.
Peter's confession of Jesus being the Christ/Messiah is the turning point in Jesus' mission. The disciples have gotten that much out of what Jesus has been teaching them. But even Peter is thinking of the Christ in popular terms: a holy warrior-king who will rout the Romans and set up a physical Kingdom of God. So he is not prepared for the truth about the cross. But that is at the heart of Jesus' mission and those who follow him must be prepared to follow him even to death.