What does Mark 8:19 mean?

At first glance, Jesus sounds like a frustrated dad who walked into the living room to see his sons jumping off of suddenly-broken furniture. However, the lesson He is trying to impart is imperative for the spread of Christianity and the establishment of the church. Some undetermined time before, Jesus and the disciples had met with five thousand men plus women and children in a plain outside of Bethsaida near the northern shore of the Sea of Galilee. He healed the sick (Matthew 14:14) and taught them until sunset. When the disciples showed concern that the people had nothing to eat, Jesus took the five loaves and two fish from a boy's lunch and turned it into enough to feed everyone. When the crowd had had its fill, the disciples gathered twelve baskets filled of bread and fish pieces (Mark 6:34–44).

Jesus is trying to warn the disciples to reject false teaching, an essential skill for the men tasked with spreading the gospel. But they also need to know how to trust God for their physical well-being. He has promised that He will give them rest from heavy labor (Matthew 11:29–30), a statement meant in part to ease fears about basic needs while following God's willHe has assured them that God sees their every move (Matthew 10:29–31). And they have seen His power and authority to provide a meal for a mob from next to nothing. They should be able to extrapolate that a generous, loving God (Matthew 7:7–11) will be able to feed them from one loaf.

After Jesus' return to heaven, the disciples will rely on the truth of God's provision as much as they rely on their ability to detect false teaching. They will be beaten (Acts 5:40), imprisoned (Acts 12:3–5), and beheaded (Acts 12:2). As they gain discernment in recognizing false teaching and beliefs, their trust in God will grow to the point where they don't fear death, let alone a hungry boat ride.