John Chrysostom

They feared that perchance they had lost the grace conferred upon them; for they had already received power over unclean spirits. It goes on: And he said unto them, This kind can come forth by nothing but by prayer and fasting.

Catena Aurea by Aquinas ↗

Theophylact of Ohrid

The disciples were afraid that they had lost the grace given to them by the Lord, and that this was why they could not cast out the demon. Notice also their reverence in that they approach Jesus privately. "This kind." Which kind? The kind of those who were demon-possessed at every new moon, or in general the entire kind of demons, which is not cast out except by prayer and fasting. Both the one who suffers from them and the one who wishes to heal must fast; both need this, but it is especially required of the sufferer himself. …

Commentary on Mark ↗

Bede

And when He had entered into the house, His disciples asked Him privately: Why could not we cast him out? And He said to them: This kind can go out by nothing but by prayer and fasting. While teaching the apostles how the most wicked demon should be expelled, He instructed all to life, so that we may know that all the gravest temptations of either unclean spirits or men are to be overcome by fasting and prayers. The wrath of the Lord, when kindled in vengeance of our sins, can also be appeased by this singular remedy. …

On the Gospel of Mark ↗