Thirdly, the Holy Ghost appeared under the form of a dove on account of the proper effect of baptism, which is the remission of sins and reconciliation with God: for the dove is a gentle creature. Wherefore, as Chrysostom says, (Hom. xii in Matth.), "at the Deluge this creature appeared bearing an olive branch, and publishing the tidings of the universal peace of the whole world: and now again the dove appears at the baptism, pointing to our Deliverer."
Fourthly, the Holy Ghost appeared over our Lord at His baptism in the form of a dove, in order to designate the common effect of baptism—namely, the building up of the unity of the Church. Hence it is written (Eph. 5:25-27): "Christ delivered Himself up . . . that He might present . . . to Himself a glorious Church, not having spot or wrinkle, or any such thing . . . cleansing it by the laver of water in the word of life." Therefore it was fitting that the Holy Ghost should appear at the baptism under the form of a dove, which is a creature both loving and gregarious. Wherefore also it is said of the Church (Cant 6:8): "One is my dove."
But on the apostles the Holy Ghost descended under the form of fire, for two reasons. First, to show with what fervor their hearts were to be moved, so as to preach Christ everywhere, though surrounded by opposition. And therefore He appeared as a fiery tongue. Hence Augustine says (Super Joan., Tract. vi): Our Lord "manifests" the Holy Ghost "visibly in two ways"—namely, "by the dove coming upon the Lord when He was baptized; by fire, coming upon the disciples when they were met together . . . In the former case simplicity is shown, in the latter fervor . . . We learn, then, from the dove, that those who are sanctified by the Spirit should be without guile: and from the fire, that their simplicity should not be left to wax cold. Nor let it disturb anyone that the tongues were cloven . . . in the dove recognize unity."
Secondly, because, as Chrysostom says (Gregory, Hom. xxx in Ev.): "Since sins had to be forgiven," which is effected in baptism, "meekness was required"; this is shown by the dove: "but when we have obtained grace we must look forward to be judged"; and this is signified by the fire. _______________________
SEVENTH ARTICLE [III, Q. 39, Art. 7]
Whether the Dove in Which the Holy Ghost Appeared Was Real?
Objection 1: It would seem that the dove in which the Holy Ghost appeared was not real. For that seems to be a mere apparition which appears in its semblance. But it is stated (Luke 3:22) that the "Holy Ghost descended in a bodily shape as a dove upon Him." Therefore it was not a real dove, but a semblance of a dove.
Obj. 2: Further, just as "Nature does nothing useless, so neither does God" (De Coelo i). Now since this dove came merely "in order to signify something and pass away," as Augustine says (De Trin. ii), a real dove would have been useless: because the semblance of a dove was sufficient for that purpose. Therefore it was not a real dove.
Obj. 3: Further, the properties of a thing lead us to a knowledge of that thing. If, therefore, this were a real dove, its properties would have signified the nature of the real animal, and not the effect of the Holy Ghost. Therefore it seems that it was not a real dove.
On the contrary, Augustine says (De Agone Christ. xxii): "Nor do we say this as though we asserted that our Lord Jesus Christ alone had a real body, and that the Holy Ghost appeared to men's eyes in a fallacious manner: but we say that both those bodies were real."