Glossa Ordinaria — as excerpted in the Catena Aurea on Mark 9:14-29

“(non occ.) Now he expresses in the words of his petition his want of faith; for that is the reason why he adds, But if thou canst do any thing, have compassion on us, and help us. For in that he says, If thou canst do any thing, he shews that he doubts His power, because he had seen that the disciples of Christ had failed in curing him; but he says, have compassion on us, to shew the misery of the son, who suffered, and the father, who suffered with him. It goes on: Jesus said unto him, If thou canst believe, all things are possible to him that believeth.”