What does Luke 9:40 mean?

A desperate father is explaining his situation to Jesus. While Jesus, Peter, James, and John were climbing down the Mount of Transfiguration, the other nine disciples were failing miserably at rescuing a young boy. The boy is the only child of his father. A demon afflicts him. The demon seems to either give the boy epileptic seizures or exacerbate the epilepsy he already has (Luke 9:37–39). What Luke does not mention is that the demon also throws the boy into water or fire, attempting to kill him (Mark 9:22).

How much do we blame the disciples for their failure? Jesus empowered them (Luke 9:1) and holds them accountable. Jesus says, "O faithless and twisted generation, how long am I to be with you and bear with you?" (Luke 9:41). This scathing remark likely refers to the crowd, the scribes, the father, and the disciples as well (Mark 9:14, 22–23). Jesus later tells the disciples they failed because of their lack of faith (Matthew 17:19–20). And yet He also says it is a particularly difficult demon to cast out, requiring faith and prayer (Matthew 17:20; Mark 9:29), corroborating the father's description of the tenacity of the evil spirit (Luke 9:39).

The truth is that Jesus expects us to at least believe what we know and live out of those beliefs. The father should have believed what he had heard about Jesus' power. The scribes should have believed what they knew about Jesus in the context of Old Testament prophecies. The disciples should have believed they still had the power to cast out demons. "Faith" implies trusting in what Jesus says because of what we already know. Those who really contemplate God's work in their lives should admit that they know more than enough to trust Jesus.