Tackling Tradition 88: Because Of Your Little Faith.,

And when they came to the crowd, a man came up to him and, kneeling before him, 15 said, “Lord, have mercy on my son, for he has seizures and he suffers terribly. For often he falls into the fire, and often into the water. 16 And I brought him to your disciples, and they could not heal him.” 17 And Jesus answered, “O faithless and twisted generation, how long am I to be with you? How long am I to bear with you? Bring him here to me.” 18 And Jesus rebuked the demon, and it came out of him, and the boy was healed instantly. 19 Then the disciples came to Jesus privately and said, “Why could we not cast it out?” 20 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.”
 (Matthew 17:14–22, ESV)

Growing up, you may have heard that the reason the disciples could not cast out the demon in this text because they lacked faith — ergo God refused to underwrite their efforts &/or the demon knew this & responded accordingly. 

I disagree.  After telling them of their little faith, Yeshua says that “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 text says that faith like the grain of a mustard seed can move mountains.   Yeshua’s focus isn’t on the disciples lack of faith —- > rather His focus is on the littleness of their faith & the power thereof.  
Consequently, the demon knew this & decided to remain out of high handed recalcitrance, & the LORD didn’t underwrite it as a check on the Disciples’ pride &/or because He wanted the focus of this event both literarily & literally (in real life) to be on Yeshua as He made His way to Jerusalem where He would confront the Sanhedrin in the temple & ultimately pay the price for having done so.   

O LORD, Hear Our Prayers!