John Chrysostom — as excerpted in the Catena Aurea on Mark 9:14-29

“(Vict. Ant. e Cat. in Marc. sed v. Chrys. ubi sup.) His meaning is; such a plenitude of virtue is there in Me, that not only can I do this, but I will make others to have that power; where fore if thou canst believe as thou oughtest to do, thou shalt be able to cure not only him, but many more. In this way then, He endeavoured to bring back to the faith, the man who as yet speaks unfaithfully. There follows, And straightway the father of the child cried out, and said with tears, Lord, I believe; help thou mine unbelief. But if he had already believed, saying, I believe, how is it that he adds, help thou mine unbelief? We must say then that faith is manifold, that one sort of faith is elementary, another perfect; but this man, being but a beginner in believing, prayed the Saviour to add to his virtue what was wanting.”