Matthew 17:16

And I brought him to thy disciples, and they could not cure him.

The latent doubt planted in the apostles' hearts by the Pharisees must surely have played a part in the inability to heal the lunatic. Their faltering faith, coupled with the double difficulty at hand, made them powerless to effect a cure. These same disciples had once returned with joy over the fact that demons were subject to them in the name of Christ (Luke 10:17ff); but then they were powerless in the presence of that lunatic boy. This indicates the difficulty the apostles had in maintaining their faith under the rising attacks of the Pharisees, the revelation that Christ would suffer death, and the temporary absence of Jesus with three of their number on the Holy Mountain.