Today's Gospel (Mt 17:14-20) contains some of the most mis-applied words of Christ in the whole Bible, and begs for a canonical reading within the context of everything else that the Bible contains. In the passage, the disciples are unable to cast out a demon and ask our Lord why. "Because of your little faith. Amen, I say to you, if you have faith the size of a mustard seed, you will say to this mountain, 'Move from here to there,' and it will move. Nothing will be impossible for you."
I know a woman who has lupus. Other members of her Pentecostal community have told her that if she just had enough faith, she wouldn't be sick. I know a man with hemophilia. He was told by a Catholic "evangelist" that if he asked to be healed with sufficient faith, that he would be healed. She still has lupus, and he still has hemophilia. I don't think that necessarily says anything about their faith or lack thereof.
There are counterexamples within the Bible indicating that those who read the passage this way are misinterpreting the text. Consider St. Paul. He wrote to the Corinthians, "Therefore, that I might not become too elated, a thorn in the flesh was given to me, an Angel of Satan, to beat me, to keep me from being too elated. Three times I begged the Lord about this, that it might leave me, be he said to me, 'My grace is sufficient for you, for power is made perfect in weakness.'" (2 Cor 7-9)
Job was full of faith, and he was blessed by God with family and possessions. But God stripped them away. He loses his property and his children, and his body is stricken with disease. Yet, Job remains faithful.
Jesus himself offers the parable of Lazarus and the rich man, in which the faithful Lazarus dies as a diseased beggar (Luke 16:19-29).
So, while today's gospel does indeed emphasize the need for and the power of faith, it should not be read in isolation from the rest of the Bible. God wants us to accomplish great things, and we must trust that he will not abandon us. As St. Paul says in Romans 8:31, "If God is for us, who can be against us?" But God often chooses to work through weakness. His ways are not our ways, and we can't always understand what He's up to. If what we ask for isn't part of his plan, we won't get what we ask for. On the other hand, if our will is attuned to his and we have faith, then nothing can stop us.