3 But I imagine, beloved brethren, that you remember that this Gospel is read in order in suitable portions; and I think that it has not escaped you what has lately been treated of, specially the recent matters concerning John and the dove. Concerning John, namely, what new thing he learned concerning the Lord by means of the dove, although he had already known the Lord. And this was discovered by the inspiration of the Spirit of God, that John indeed already knew the Lord, but that the Lord Himself was to baptize, that the power of baptizing He would not transfer from Himself to any one, this he learned by means of the dove, because it was said to him, “On whom you shall see the Spirit descending as a dove, and abiding upon Him, this is He which baptizes with the Holy Ghost.” What is “This is He”?
Not another, although by means of another. But why by means of a dove? Many things were said, and I am not able, nor is there need that I should go over all—principally, however, to denote peace, because also the trees which were baptized outside, because the dove found in them fruit, it brought to the ark, as you remember the dove sent out by Noah from the ark, which floated on the flood and was washed by baptism, was not submerged. When, then, it was sent forth, it brought an olive branch; but it had not leaves alone, it had also fruit. This, then, we ought to wish for our brethren who are baptized outside, that they may have fruit; the dove will not permit them to remain outside, but bring them back to the ark.
For the whole of fruit is charity, without which a man is nothing, whatever else he have. And this, which is most fully said by the apostle, we have mentioned and recounted. For he says, “Though I speak with the tongues of men and of angels, and have not charity, I have become as sounding brass or a tinkling cymbal; and though I should have all knowledge, and know all mysteries, and have all prophecy, and should have all faith” (but in what sense did he say all faith?), “so that I could remove mountains, and have not charity, I am nothing.
And though I should distribute all my goods to the poor, and though I should give my body to be burned, and have not charity, it profits me nothing.” But in no manner are they able to say that they have charity who divide unity. These things were said: let us see what follows.
Source: Tractates on the Gospel of John (New Advent)