Little faith: The father of the boy addresses Jesus as “Lord” and requests that he have pity on his son, for he is a lunatic and suffers terribly. Matthew states that the man kneels down before Jesus, thus portraying the father as a believer—he kneels (worships) and begs for mercy by addressing Jesus as “Lord” instead of the usual “Teacher.”
The father refers to his son as a “lunatic,” literally, “one affected or struck by the moon.” Matthew reflects the belief of ancient physicians that the phases of the moon affected epileptic seizures. Jesus’ disciples have not been able to cure the boy. This inability is credited to their weak faith.
At the same time, Jesus takes the occasion to rebuke the present generation for its faithlessness. Matthew is alerting the members of his church that the ministry of Jesus is drawing to a close (in fact, it is always drawing to a close), and that the time to profess faith in Jesus is now.
In private, the disciples learn that all it takes is faith the size of a mustard seed. For Matthew, authentic faith can move mountains.
The disciples in Matthew’s community whose faith is not as deep as it should be are at one and the same time being warned and encouraged. They are being warned not to abandon the little faith they have. They are being encouraged to use it to move mountains (hyperbolically!)