bewailing sins.

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.
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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.