John Chrysostom

(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. …

Catena Aurea by Aquinas ↗

Theophylact of Ohrid

So that man, having come to Jesus, blamed the disciples for being unable to heal his son; but Christ turns the accusation back upon the man himself and speaks as if to say: you have no faith, and therefore you yourself are to blame that your son was not healed... However, He does not stop merely at reproach, but also grants healing, displaying in the healing of the boy not haughtiness, but much more — humility. …

Commentary on Mark ↗

Irenaeus

And not merely in works, but also in faith, has God preserved the will of man free and under his own control, saying, "According to thy faith be it unto thee;" thus showing that there is a faith specially belonging to man, since he has an opinion specially his own. And again, "All things are possible to him that believeth;" and, "Go thy way; and as thou hast believed, so be it done unto thee." Now all such expressions demonstrate that man is in his own power with respect to faith. …

Irenaeus Against Heresies Book 4 ↗

Bonaventure

From the understanding of faith there comes forth in the soul a threefold flow out of this light, through which it is steadied in God. The first is the strength of virtue: nothing is difficult to the soul that has an unperturbed faith, for "If thou canst believe, all things are possible to him who believes." Indeed, he is ready to sustain anything for God's sake, since virtue is made steady by faith.

Collations on the Hexaemeron, Collation 9 ↗