6 And the Spirit of God was borne upon the face of the waters. Does this spirit mean the diffusion of air? The sacred writer wishes to enumerate to you the elements of the world, to tell you that God created the heavens, the earth, water, and air and that the last was now diffused and in motion; or rather, that which is truer and confirmed by the authority of the ancients, by the Spirit of God, he means the Holy Spirit. It is, as has been remarked, the special name, the name above all others that Scripture delights to give to the Holy Spirit, and always by the spirit of God the Holy Spirit is meant, the Spirit which completes the divine and blessed Trinity.
You will find it better therefore to take it in this sense. How then did the Spirit of God move upon the waters? The explanation that I am about to give you is not an original one, but that of a Syrian, who was as ignorant in the wisdom of this world as he was versed in the knowledge of the Truth. He said, then, that the Syriac word was more expressive, and that being more analogous to the Hebrew term it was a nearer approach to the scriptural sense. This is the meaning of the word; by “was borne” the Syrians, he says, understand: it cherished the nature of the waters as one sees a bird cover the eggs with her body and impart to them vital force from her own warmth.
Such is, as nearly as possible, the meaning of these words— the Spirit was borne: let us understand, that is, prepared the nature of water to produce living beings: a sufficient proof for those who ask if the Holy Spirit took an active part in the creation of the world.
Source: Nine Homilies of Hexaemeron (New Advent)