The disciples had failed to cast out a demon and Jesus had rebuked them for being unbelieving. He said they were part of a religious generation which did not trust the Lord (Matthew 17:14-18). Continuing the discussion with Him privately, they wanted to know why they could not heal the boy, when they had cast out demons and healed previously (Matthew 10:1).
 
The reason for spiritual ineffectiveness is unbelief (Matthew 17:17). They had not been authorised by Jesus to perform this healing and so they only had their own resources and experience to rely upon. They were not relying on Jesus. Without His power, they were powerless. And without His Spirit they were as weak as the boy they were trying to help (the Spirit was not given until Pentecost after Jesus' ascension).
 
These verses do not mean that 'faith' is belief in your power, belief in your experience, belief in your method or even belief in your faith. Faith can never be placed in a power, system or process because faith is relational; it applies only to trusting a person. The disciples did not realise that the power to heal was not resident in them but only in Jesus. That did not enter their minds at all. They did not even have the smallest grain of faith that the power of Jesus might operate to heal the boy.
 
These verses have been much misunderstood and abused. Some Christians have tried all kinds of psychological devices to try to 'generate' faith or 'feel' faith over matters which God has never authorised; or have accused others of faithlessness because their prayers have not been answered in the way they hoped. Faith is simply a matter of trusting the Lord to do His will, and working with Him in matters which He has authorised. It does not need loud noises, stoic self-imposed suffering or exaggerated claims. Faith is the confidence that God will fulfil His promises. Only a little of that kind of faith is needed because God will honour it and take whatever action He knows is right.