On Jesus' return from the mountain of Transfiguration (17:1-13), he is greeted by a hysteric dad, kneeling and begging Jesus to heal his son. Nine of Jesus' disciples had failed to perform the miracle. In v. 17, Jesus laments over the faithlessness of His followers. Their faith is actually painful to Him; He is disgusted by it. His soul aches as he sees the poverty and perverseness of their faith. Somehow they had forgotten their authority to cast out demons and heal the hurting came from God. Probably, they were thinking they had somehow become magicians or powerful healers in their own right. Faith is simply "dependence on one who is stronger." But the disciples were no longer dependent on God, but were ready to tackle the world on their own.
Jesus wanted them to have simple, mustard-like faith in Him (prayerful dependence, see Mark 9:29). When someone's faith is based on the all-mighty, all-knowing and all-loving Creator of the universe, even a mustard-size (think kid's size) amount of faith is sufficient to change the world; no ministry is impossible with faith and dependence in the great God of the universe (v. 21). He is the great God who came in the person of Jesus Christ who went to Calvary to die for faithless people, and yet His own faith was rewarded and He rose victorious over sin, death, and hell (vv. 22-23).
This transition from faithful to faithless is common among the most "mature" followers of Jesus Christ. After a few years of service, we look back and see all kinds of miraculous happenings. We see that we have changed lives, spoken words of wisdom, and been blessed with financial blessings, etc. The sad thing (and painful to God) is we begin to forget our dependence on God and start thinking these blessings have all come about through our own brilliance or power. Like the disciples, the Lord will bring us through our own painful seasons of failure and loss to bring us back to Him. It's the most loving thing God does sometimes, when He removes His hand of blessing so we come and kneel before Him confessing our pride and idolatry. God is to get the glory; we are to receive His grace.