2 For the fact that the Holy Spirit appeared in bodily form, as a dove, was a sight begun and ended at the time: just as also, when He descended upon the disciples, there were seen upon them cloven tongues as of fire, which also sat upon every one of them. Any one, therefore, who says that the dove was connected with the Holy Spirit in the unity of His person, as that it and Godhead (for the Holy Spirit is God) should go to constitute the one person of the Holy Spirit, is compelled also to affirm the same thing of that fire; and so may understand that he ought to assert neither.
For those things in regard to the substance of God, which needed at any time to be represented in some outward way, and so exhibited themselves to men's bodily senses, and then passed away, were formed for the moment by divine power from the subservient creation, and not from the dominant nature itself; which, ever abiding the same, excites into action whatever it pleases; and, itself unchangeable, changes all things else at its pleasure. In the same way also did that voice from the cloud actually strike upon the bodily ears, and on that bodily sense which is called the hearing; and yet in no way are we to believe that the Word of God, which is the only-begotten Son, is defined, because He is called the Word, by syllables and sounds: for when a sermon is in course of delivery, all the sounds cannot be pronounced simultaneously; but the various individual sounds come, as it were, in their own order to the birth, and succeed those which are dying away, so that all that we have to say is completed only by the last syllable.
Very different from this, surely, is the way in which the Father speaks to the Son, that is to say, God to God, His Word. But this, so far as it can be understood by man, is a matter for the understanding of those who are fitted for the reception of solid food, and not of milk. Since, therefore, the Holy Spirit became not man by any assumption of humanity, and became not an angel by any assumption of angelic nature, and as little entered into the creature-state by the assumption of any creature-form whatever, how, in regard to Him, are we to understand those words of our Lord, “For He shall not speak of Himself; but whatsoever He shall hear, that shall He speak”?
A difficult question; yea, too difficult. May the Spirit Himself be present, that, at least up to the measure of our power of thinking on such a subject, we may be able to express our thoughts, and that these, according to the little measure of my ability, may find entrance into your understanding.
Source: Tractates on the Gospel of John (New Advent)