A Spirit Cast Out of the Boy (Mark's Gospel)

How the problems of discipleship are shown in the Spirit Cast Out of the Boy:

The disciples are unable to cast out the demon from the boy and this leads to them arguing amongst one another. As a disciple, one must be prepared to not understand Jesus' teachings or not to immediately be able to do the things he asks. 

- The disciples inability to cure the man could have been down to lack of faith and belief in God, as Jesus says to the apostles "how unbelieving...you...are". If this is the case, it shows that a problem of discipleship is knowing when one has enough faith. How are disciples (including those in the time the gospel was written, as well as today) to know when they have enough faith to do things. 

- Jesus says that "only prayer" could "drive" out the demons. Seeing as all people can pray, including Jesus' disciples, why did they, Jesus' most trusted and instructed disciples, fail? Is the problem that disciples are inadequate of doing what Jesus says? Or is it that disciples are unable to pray?

- In spite of being clear instructions and having the power and authority bestowed upon them by Jesus, the disciples were incapable of fulfilling a task set by Jesus. If disciples could not follow Jesus' instructions through direct addressing (explanation from him himself), does this mean that disciples are unable to fulfill a task? Will we definitely be unable to fulfill…