Clement of Alexandria

Therefore God does not here take the semblance of man, but of a dove, because He wished to show the simplicity and gentleness of the new manifestation of the Spirit by the likeness of the dove. For the law was stern, and punished with the sword; but grace is joyous, and trains by the word of meekness. Hence the Lord also says to the apostles, who said that He should punish with fire those who would not receive Him, after the manner of Elias: "Ye know not what manner of spirit ye are of."

Fragments Found in Greek Only in the Oxford Edition ↗

Ambrose of Milan

But the Lord is not moved against them, that He might show that perfect virtue has no feeling of revenge, nor is there any anger where there is fulness of love. For weakness must not be thrust out, but assisted. Let indignation be far from the religious, let the high-souled have no desire of vengeance. Hence it follows, But he turned and rebuked them, and said, Ye know not what manner of spirit ye are of.

Catena Aurea by Aquinas ↗

Bede

But when his disciples James and John saw this, they said, "Lord, do you want us to command fire to come down from heaven and consume them?" And he turned and rebuked them, and said, "You do not know what kind of spirit you are of. Great and holy men, who already knew very well that this death, which separates the soul from the body, is not to be feared, according to their spirit who feared it, punished some sins with death, so that fear might be instilled in the living, and to those who were punished with death, death itself would …

On the Gospel of Luke ↗

Bonaventure

Secondly, as regards the reasonable reprehension in the word of the Lord, it is added: And turning, he rebuked them, saying: You do not know of what spirit you are. He says this because they believed themselves to be moved by a spirit of righteousness, yet they were moved by a zeal of bitterness, which they ought not to have been, as James 3 says: "But if you have bitter zeal, and there are contentions in your hearts, do not glory and be liars against the truth." And therefore it is said in 1 John 4: "Do not believe every spirit, but …

Commentary on Luke, Chapter 9 ↗