“And Jesus said unto them, Because of your unbelief: for verily I say unto you, If ye have faith as a grain of mustard seed, ye shall say unto this mountain, Remove hence to yonder place; and it shall remove; and nothing shall be impossible unto you.”
He said to them: because of your little faith. For truly, I say to you, if you have faith like a grain of mustard seed, you will say to this mountain, move from here to there, and it will move, and nothing will be impossible for you. The mustard seed faith — the smallest faith that is genuinely faith — moves mountains. The contrast between the disciples' little faith (which failed to cast out the demon) and the mustard seed faith (which moves mountains) communicates that the issue is not the quantity of faith but its quality: genuine faith, however small, accesses divine power. The nothing will be impossible for you is the promise of prayer in chapter 18.
Church Fathers on Matthew 17:20
The disciples had received from the Lord the power over unclean spirits, and when they could not heal the daemoniac thus brought to them, they seem to have had misgivings lest they had forfeited the grace once given to them; hence their question. And they ask it apart, not out of shame, but because of the unspeakable matter of which they were to ask.
The Apostles had believed, yet their faith was imperfect; while the Lord tarried in the mount, and they abode below with the multitude, their faith had become stagnant.
This is what the Lord says in another place, 'Whatsoever ye shall ask in my name believing, ye shall receive.' [Matt 21:22, John 16:23] Therefore when we receive not, it is not the weakness of Him that gives, but the fault of them that ask.