Titus of Bostra

He might indeed have healed him by His simple command, but He makes his sufferings public, bringing the weak in faith to the sight of things present. Then the devil, when he perceived our Lord, rends and dashes the child clown; as it follows, And as he was yet a coming, the devil threw him down, and tare him; that so first the sufferings should be made manifest, then the remedy be applied.

Catena Aurea by Aquinas ↗

Bede

While the boy is coming to our Lord, he is dashed to the ground; because men when turned to the Lord are often grievously afflicted by the devil, that he may instil a hatred of virtue, or revenge the injury of his expulsion. As in the beginning of the Church he waged as many fierce conflicts as he had to bewail losses suddenly brought upon His kingdom. …

Catena Aurea by Aquinas ↗

Bonaventure

And because "there is no agreement of Christ with Belial, nor of light with darkness," therefore he adds: And as he was approaching, the demon threw him down and convulsed him: in which it is shown that he came unwillingly to the light as one doing evil, according to John three: "He who does evil hates the light." At the same time it is also shown that the devil most greatly opposes anyone possessed by him from being converted to Christ, as Bede says in the Gloss: "Whence the boy approaching the Lord is thrown down, because those …

Commentary on Luke, Chapter 9 ↗