St. Cyprian (+258), Bishop of Carthage (in modern Tunisia), a skillful Latin rhetorician and writer, and finally, martyr, who, upon being condemned to death by the sword replied, "Thanks be to God!", writes in his The Unity of the Church.
It is said that the Holy Spirit appeared in the form of a dove because the dove is a simple harmless animal, not bitter with gall, not savage with its bites, not fierce with rending talons; it loves the dwellings of men, is able to live together in one nest, together it raises its young, they remain together when they fly, spend their life in mutual association, signify the concord of peace with the kiss of their bill, and fulfill the law of harmony in all things."
St. Thomas Aquinas (+1274), a millennium later, in his "Commentary on the Gospel of St. John", confirms these theological insights: "Many reasons are given why the Holy Spirit appeared as a dove rather than in some other form.
Firstly, because of its simplicity, for the dove is simple: "Be wise as serpents, and innocent as doves" (Mt 10:16). And the Holy Spirit, because he inclines souls to gaze on one thing, that is, God, makes them simple.
Secondly, because of the unity of charity; for the dove is much aglow with love: "my dove, my perfect one" (Song 6:9). So, in order to show the unity of the Church, the Holy Spirit appears in the form of a dove.
Thirdly, because of its groaning, for the dove has a groaning chant; so also the Holy Spirit "intercedes with sighs too deep for words" (Rom 8:26).
Fourthly, because of the dove's fertility, for the dove is a very prolific animal. And so in order to signify the fecundity of spiritual grace in the Church, the Holy Spirit appeared in the form of a dove.
Fifthly, because of its cautiousness. For it rests upon watery brooks, and gazing into them can see the hawk flying overhead and so save itself. And so, because our refuge and defense is found in baptism, the Holy Spirit appropriately appeared in the form of a dove.
Like a dove. What more can be said?
Yours in our Lord upon whom descended the Holy Spirit,