Isaiah was sent by the Spirit, and accordingly the same Spirit was seen by him. What is meant by the revolving wheels, and the various wings, and how since the Spirit is proclaimed Lord of Sabaoth by the Seraphim, certainly none but impious men can deny Him this title.
159. Since, then, He has a share in the kingdom, what hinders us from understanding that it was the Holy Spirit by Whom Isaiah was sent? For on the authority of Paul we cannot doubt, whose judgment the Evangelist Luke so much approved in the Acts of the Apostles as to write as follows in Paul's words: “Well spoke the Holy Spirit through Isaiah the prophet to our fathers, saying: Go to this people and say, You shall hear with the ear and shall not understand, and seeing you shall see and shall not perceive.”
160. It is, then, the Spirit Who sent Isaiah. If the Spirit sent him, it is certainly the Spirit Whom, after Uzziah's death, Isaiah saw, when he said: “I saw the Lord of Sabaoth sitting upon a throne, high and lifted up, and the house was full of His majesty. And the Seraphim stood round about Him, each one had six wings, and with two they were covering His face, and with two they were covering His feet, and with two they were flying; and they cried out one to the other, and said, Holy, holy, holy is the Lord of Sabaoth, the whole earth is full of His majesty.”
161. If the Seraphim were standing, how were they flying? If they were flying, how were they standing? If we cannot understand this, how is it that we want to understand God, Whom we have not seen?
162. But as the prophet saw a wheel running within a wheel (which certainly does not refer to any appearance to the bodily sight, but to the grace of each Testament; for the life of the saints is polished, and so consistent with itself that later portions agree with the former). The wheel, then, within a wheel is life under the Law, life under grace; inasmuch as Jews are within the Church, the Law is included in grace. For he is within the Church who is a Jew secretly; and circumcision of the heart is a sacrament within the Church. But that Jewry is within the Church of which it is written: “In Jewry is God known;” therefore as wheel runs within wheel, so in like manner the wings were still, and the wings were flying.
163. In like manner, too, the Seraphim were veiling His face with two wings, and with two were veiling His feet, and with two were flying. For here also is a mystery of spiritual wisdom. Seasons stand, seasons fly; the past stand, the future are flying, and like the wings of the Seraphim, so they veil the face or the feet of God; inasmuch as in God, Who has neither beginning nor end, the whole course of times and seasons, from this knowledge of its beginning and its end, is at rest. So, then, times past and future stand, the present fly. Ask not after the secrets of His beginning or His end, for there is neither. You have the present, but you must praise Him, not question.
164. The Seraphim with unwearied voices praise, and do you question? And certainly when they do this they show us that we must not sometimes question about God, but always praise Him. Therefore the Holy Spirit is also the Lord of Sabaoth. Unless perchance the Teacher Whom Christ chose pleases not the impious, or they can deny that the Holy Spirit is the Lord of powers, Who gives whatever powers He Himself wills.
Source: On the Holy Spirit (New Advent)