What does Luke 12:1 mean?
Jesus is not merely condemning the Pharisees, but applying His lesson to teach how the disciples should minister to people. The Pharisees perform their religion for show. They "love the best seat in the synagogues and greetings in the marketplaces" (Luke 11:43). But they deny the power of the Holy Spirit in Jesus' works and claim He serves Satan (Luke 11:14–23). They lead the people into an ironic combination of legalism and sin, and ultimately death (Matthew 23:15; Luke 11:42–44).Meanwhile, Jesus' following grows. This is not because the people realize His words have life, but because He feeds and heals them (John 6:26). Had they intended to learn, they would make space and listen. Instead, they trample each other, trying to touch Him to get healing or to get His attention to make judgments on their squabbles (Luke 12:13–21).
"Leaven" is any ingredient, such as baking soda, baking powder, or primarily yeast, which makes bread rise. The Bible uses it as a metaphor for anything that permeates the whole, as a little leavening has effect over a large batch of dough. In at least one parable, Jesus uses leavening as an illustration of how the kingdom of God can spread (Luke 13:20–21). Most of the time, however, leavening is used as a symbol of sin. A little sin can spread and cause great damage. The teaching of the Pharisees about Jesus will spread until the Sanhedrin is able to rally a crowd to demand that Jesus be crucified (Matthew 27:20–23).
"First" means that despite being overwhelmed by a crowd of thousands, Jesus is prioritizing the education of the disciples. That is what the "travelogue" section of the Gospel of Luke involves. Jesus is training the disciples to explain the need for the crucifixion before He experiences it in Jerusalem, so they are equipped to build the church. First, Jesus wants to teach the disciples to be bold in the face of those who would persecute and kill them for remaining faithful to Him (Luke 12:1–12). They must focus on their tasks until He returns (Luke 12:35–48). They should even be willing to give up family if necessary (Luke 12:49–53). Jesus keeps His focus on the disciples despite an interruption from the crowd (Luke 12:13) before addressing the crowd's need to recognize the kingdom of God is coming and their need to reconcile with each other and with God (Luke 12:54—13:9).