Note 2 at Mt 17:20: The disciples' unbelief in this instance was not a disbelief that God's power could produce deliverance (see note 1 at Lu 9:41), but rather, it was a "natural" kind of unbelief that came from hardened hearts (see note 10 at Mr 6:52) that were more sensitive to what they saw (Mr 9:20) than to what they believed.

Note 3 at Mt 17:20: Jesus did not say that the reason these disciples could not cast this demon out was because they didn't have faith but rather because they had unbelief. On the contrary, Jesus stated that a very small amount of faith (the size of a "mustard seed") was sufficient to remove a mountain if no unbelief was present to hinder it.

Most of us have the concept that we either have faith or unbelief but that we can't have both at the same time. However, Jesus told Jairus to "believe only" (Lu 8:50) implying that faith and fear can operate in us at the same time. This was also the reason James said not to be double-minded (Jas 1:5-8). We can be thinking faith yet having thoughts of unbelief at the same time.

As explained in note 1 at Mt 17:19, these disciples did have faith to cast this demon out, and they exercised that faith but didn't see the same results that they had seen before. This was why they were concerned and asked Jesus what the problem was. They wouldn't have asked if they had not believed. They did have faith, the same faith that had effected many other deliverances. The problem wasn't their faith but rather their unbelief.

Every believer has been given "the measure of faith," but unbelief negates it. It's like hooking a team of horses up to a heavy weight and having them pull it. The weight will move. But if an equal team of horses is hooked up to the same weight and pull in the opposite direction at the same time, although great force may be exerted, the weight won't move. Likewise, unbelief counterbalances our faith. If we will just remove the unbelief, a mustard-seed amount of faith will be sufficient to move our problems.

Instead of trying to build huge amounts of faith to overcome our fears and unbelief, a simpler method is to remove our fears by cutting off their source; then our simple, "child-like" faith that remains will do the job. It doesn't take big faith, just pure faith.

Those of us who tolerate high levels of unbelief in our lives will never be able to build our faith big enough to overcome unbelief's negative force. The only way we can receive is to get others to mix their faith with ours or draw on one of the supernatural ministry gifts in someone else, such as the gift of faith (1Co 12:9). God's best is for us to receive directly from Him. We will only be successful at that when we not only build our faith but also destroy our doubts. (See note 10 at Mr 6:52.)