This miracle teaches that it takes the power of faith to overcome the enemy (Matthew 17:20; Mark 9:19, 23-24). Why had the nine disciples failed? They had been careless in their personal spiritual walks and had neglected prayer and fasting (Mark 9:29). The authority that Jesus had given them was effective only if exercised by faith, but faith must be cultivated through spiritual discipline and devotion. The apostle Paul uses Abraham as an example of this:
No unbelief or distrust made him waver (doubtingly question) concerning the promise of God, but he grew strong and was empowered by faith as he gave praise and glory to God, fully satisfied and assured that God was able and mighty to keep His word and to do what He had promised. That is why his faith was credited to him as righteousness (right standing with God). (Romans 4:20-22, The Amplified Bible)
Half of the healings Christ performed happened due to the prayer of loved ones and friends. The father of the demon-possessed boy remains a monument of faith, timid yet true, through his love for his boy. His pleading, "Have compassion on us, and help us," was highly honored by Jesus.
The key to the exercise of such faith is not its quantity, but the God to whom it is directed; therefore, even the smallest faith—"like a mustard seed"—will see spectacular results (Luke 17:5-6; Matthew 17:20). Jesus says in Mark 9:23, "All things are possible to him who believes." Faith acts as an open door into a relationship with God and as a shield that protects God's people when they are under spiritual attack.