John Chrysostom

The Scripture indicates that this man was extremely weak in faith, as is evident at four points: from Christ's saying that "All things are possible to him that believes;" from the saying of the man himself as he approached, "Help me in my unbelief;" from Christ's commanding the devil to "enter no more into him;" and from the man's saying again to Christ, "If you can."

The Gospel Of St Matthew, Homily 57.3 ↗

Theophylact of Ohrid

Speaking in a figurative sense, the demon casts people into the fire of anger and lust and into the stormy waves of worldly affairs.

Commentary on Mark ↗

Cyprian

That faith is of advantage altogether, and that we can do as much as we believe. In Genesis: "And Abraham believed God, and it was counted unto him for righteousness." Also in Isaiah: "And if ye do not believe, neither shall ye understand." Also in the Gospel according to Matthew: "O thou of little faith, wherefore didst thou doubt? …

Treatise XII Three Books of Testimonies Against the Jews ↗

Bede

Or by this demoniac are signified those, who are bound by the guilt of original sin, and coming into the world as criminals, are to be saved by grace; and by fire is meant the heat of anger, by water, the pleasures of the flesh, which melt the soul by their sweetness. But He did not rebuke the boy, who suffered violence, but the devil, who inflicted it, because he who desires to amend a sinner, ought, whilst he exterminates his vice by rebuking and cursing it, to love and cherish the man.

Catena Aurea by Aquinas ↗

Glossa Ordinaria

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 shows 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 show the misery of the son, who suffered, and the father, who suffered with him. …

Catena Aurea by Aquinas ↗