"I love you Lord, my strength, my God, the Rock where I take refuge."
Like the psalmist, the father of the epileptic boy took refuge in Jesus in His desperate situation which the disciples were unable to handle. Jesus chided the disciples on their "little faith". Hadn't he mandated them to cure, heal and rid people of all illness (Mt. 10)? The power to heal and exorcise was not of their own merit but a gift which is very much related to their own faith in Jesus and his saving work.
In striking contrast the father of the sick boy 'went down on his knees before him' in faith (v.14) thus enhancing the power of trusting faith. What do we do if our prayer at times seems to go unanswered? If our "faith" is controlled by remarkable/strange happenings then it isn't faith but superstition.
The disciples had been in the limelight along with Jesus during the multiplication of the loaves (Mt. 16). They had further glimpsed heaven on Mount Tabor and now on their descent from the mountain needed to be shaken out of their day-dreams and be brought back to reality on earth. The stern rebuke in verse 17 seems to be directed at the disciples and this is confirmed in verse 20 where they are criticized. Yet in the 'absence' of Jesus the disciples are promised they would achieve the impossible even if their faith was merely the size of a mustard seed.
All things are possible with God.