3 Therefore, when He sent the Holy Spirit He manifested Him visibly in two ways— by a dove and by fire: by a dove upon the Lord when He was baptized, by fire upon the disciples when they were gathered together. For when the Lord had ascended into heaven after His resurrection, having spent forty days with His disciples, and the day of Pentecost being fully come, He sent unto them the Holy Spirit as He had promised. Accordingly the Spirit coming at that time filled the place, and there was first a sound from heaven as of a rushing mighty wind, as we read in the Acts of the Apostles, and “there appeared unto them,” it says, “cloven tongues as of fire, and it sat upon each of them; and they began to speak with tongues, as the Spirit gave them utterance.” Here we have seen a dove descending upon the Lord; there, cloven tongues upon the assembled disciples: in the former, simplicity is shown; in the latter, fervency.
Now there are who are said to be simple, who are only indolent; they are called simple, but they are only slow. Not such was Stephen, full of the Holy Ghost: he was simple, because he injured no one; he was fervent, because he reproved the ungodly. For he held not his peace before the Jews. His are those burning words: “You stiff-necked and uncircumcised of heart and ears, you do always resist the Holy Spirit.” Mighty impetuosity; but it is the dove without gall raging. For that you know that he was fierce without gall, see how, upon hearing these words, they who were the ravens immediately took up stones and rushed together upon this dove.
They begin to stone Stephen; and he who a little before stormed and glowed with ardor of spirit,— who had, as it were, made an onset on his enemies, and like one full of violence had attacked them in such fiery and burning words as you have heard, “You stiff-necked and uncircumcised in heart and ears,” that any one who heard those words might fancy that Stephen, if he were allowed, would have them consumed at once—but when the stones thrown from their hands reached him, with fixed knee he says, “Lord, lay not this sin to their charge.” He held fast to the unity of the dove.
For his Master, upon whom the dove descended, had done the same thing before him; who, while hanging on the cross, said, “Father, forgive them, for they know not what they do.” Wherefore by the dove it is shown that they who are sanctified by the Spirit should be without guile; and that their simplicity should not continue cold is shown us by the fire. Nor let it trouble you that the tongues were divided; for tongues are diverse, therefore the appearance was that of cloven tongues. “Cloven tongues,” it says, “as of fire, and it sat upon each of them.” There is a diversity of tongues, but the diversity of tongues does not imply schisms. Be not afraid of separation in the cloven tongues; in the dove recognize unity.
Source: Tractates on the Gospel of John (New Advent)