In his book The Covet War, Howard Pittman reflected on the type of faith that is needed to deliver from demons:
Matthew 17:20. “And Jesus said unto the, 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 younder place; and it shall remove; and nothing shall be impossible unto you.”
Jesus was herein talking to his disciples and accused them of not being believers because they had just failed to cast out an evil spirit that was in a little boy. Back in Luke 9:1, they had heard with their own ears, the authority that Jesus had spoken with his own lips, over all unclean spirits. (“Then he called his twelve disciples together, and gave them power and authority over all devils, and to cure diseases.”)
Until Matthew 17, that authority had worked. In each case, where the disciples had commanded an unclean spirit to come out of a person, it had obeyed! (“And the seventy returned again with joy, saying, Lord, even the devils are subject unto us through thy name. “) Luke 10:17.
So, how come this one devil refused to obey them, if they had the authority?
Because they were believers only so long as they did not doubt! Until the case of the little boy, all demon spirits had responded to their first command. In Matthew 17, in the case of the little boy, this was a different kind of spirit (Howbeit THIS KIND goeth not out but by prayer and fasting”).
Matthew 17:21 and required more commitment on the part of the disciples. When the spirit did not respond to their first command, they doubted, thus becoming unbelievers. When one doubts, one is not a believer. (Matthew 17:20) In verse 20 Jesus told the disciples why they could not cast out the spirit. (Because of their unbelief). In verse 21 he told them why their unbelief had manifested. (A different kind of Spirit than they had run into.) This particular spirit did not respond to a first command.
Verse 21 showed that a commitment over a period of time was needed to deal with this spirit. Jesus was not condemning his disciples for not fasting, as some people claim, because they could not fast, (Matthew 9:14-15) (14) “Then came to him the ·disciples of John, saying, why do we and the Pharisees fast oft, but thy disciples fast not? ( 15) And Jesus said unto them, can the children of the bridechamber mourn, as long as the bridegroom is with them? But the days will come, when the bridegroom shall be taken from them, and then they shall fast.” Their failure to deal with the spirit in the little boy was because they lost their authority by doubting! A good rule to remember is that 2 ounces of doubt can destroy 12 pounds of faith!