17:13. Then the disciples understood, that he had spoken to them of John the Baptist.
17:14. And when he was come to the multitude, there came to him a man falling down on his knees before him saying: Lord, have pity on my son, for he is a lunatic, and suffereth much: for he falleth often into the fire, and often into the water.
17:15. And I brought him to thy disciples, and they could not cure him.
17:16. Then Jesus answered and said: O unbelieving and perverse generation, how long shall I be with you? How long shall I suffer you? Bring him hither to me.
17:17. And Jesus rebuked him, and the devil went out of him, and the child was cured from that hour.
17:18. Then came the disciples to Jesus secretly, and said: Why could not we cast him out?
17:19. Jesus said to them: Because of your unbelief. For, amen I say to you, if you have faith as a grain of mustard seed, you shall say to this mountain: Remove from hence hither, and it shall remove: and nothing shall be impossible to you.
As a grain of mustard seed.... That is, a perfect faith; which in its properties, and its fruits, resembles the grain of mustard seed, in the parable, chap. 13. 31.
17:20. But this kind is not cast out but by prayer and fasting.
17:21. And when they abode together in Galilee, Jesus said to them: The Son of man shall be betrayed into the hands of men: