John Chrysostom

The Scriptures declare that this man was weak in faith, for Christ says, O faithless generation: and He adds, If thou canst believe. But although his want of faith was the cause of their not casting out the devil, he nevertheless accuses the disciples; wherefore it is added, And I spake to thy disciples that they should cast him out; but they could not. …

Catena Aurea by Aquinas ↗

Theophylact of Ohrid

This man was weak in faith, as the Lord testifies, having said: "O faithless generation," and again "all things are possible to him who believes"; and he himself also says: "help my unbelief." He also speaks against the disciples (of Jesus), as though they were all unbelievers. But he ought not to have accused them before everyone, but privately, in secret.

Commentary on Mark ↗

Bede

And he asked them: What are you discussing among yourselves? And one from the crowd answered: Teacher, I brought my son to you, who has a mute spirit; and wherever it seizes him, it throws him down, and he foams, and grinds his teeth, and becomes rigid. And I spoke to your disciples to cast it out, and they could not. As for this demoniac whom the Lord cured when descending from the mountain, Mark indeed recounts him as being deaf and mute, but Matthew recalls him as being a lunatic. …

On the Gospel of Mark ↗