Theophylact of Ohrid

Not out of any rivalry or envy does the son of thunder forbid that man from casting out demons, but he desires that all who invoke the name of Christ should also follow Christ, and that all the disciples should constitute one body. At the beginning of the evangelical preaching, it happened that some, driven by the passion of vainglory, desired to perform signs; but seeing how powerful the name of Jesus was, they invoked it and in this way performed signs, even though they were strangers to and unworthy of the grace of God. …

Commentary on Mark ↗

Augustine of Hippo

But "whatsoever is without the Catholic Church is mere pretense," yet only in so far as it is not Catholic. But there may be something Catholic outside the Catholic Church, just as the name of Christ could exist outside the congregation of Christ, in which name he who did not follow with the disciples was casting out devils. For there may be pretense also within the Catholic Church, as is unquestionable in the case of those "who renounce the world in words and not in deeds," and yet the pretense is not Catholic. …

On Baptism, Against The Donatists 7.39 (76) ↗

Bede

John answered him, saying: Master, we saw someone casting out demons in your name, who does not follow us, and we forbade him. John, loving the Lord with supreme devotion and therefore worthy of being loved in return, thought that he who does not use the office should be excluded from the benefit, but it is taught that no one should be barred from the good which he partially possesses, but rather should be encouraged towards that which he does not yet have. …

On the Gospel of Mark ↗