Macarius Chrysocephalus: Oration VIII. On Matt. viii., and Book VII. On Luke xiii.
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: “You know not what manner of spirit you are of.”
Source: Fragments (New Advent)