Although we are baptized with water and the Spirit, the latter is much superior to the former, and is not therefore to be separated from the Father and the Son.
76. There are, however, many who, because we are baptized with water and the Spirit, think that there is no difference in the offices of water and the Spirit, and therefore think that they do not differ in nature. Nor do they observe that we are buried in the element of water that we may rise again renewed by the Spirit. For in the water is the representation of death, in the Spirit is the pledge of life, that the body of sin may die through the water, which encloses the body as it were in a kind of tomb, that we, by the power of the Spirit, may be renewed from the death of sin, being born again in God.
77. And so these three witnesses are one, as John said: “The water, the blood, and the Spirit.” One in the mystery, not in nature. The water, then, is a witness of burial, the blood is a witness of death, the Spirit is a witness of life. If, then, there be any grace in the water, it is not from the nature of water, but from the presence of the Holy Spirit.
78. Do we live in the water or in the Spirit? Are we sealed in the water or in the Spirit. For in Him we live and He Himself is the earnest of our inheritance, as the Apostle says, writing to the Ephesians: “In Whom believing you were sealed with the Holy Spirit of promise, Who is an earnest of our inheritance.” So we were sealed by the Holy Spirit, not by nature, but by God, for it is written: “He Who anointed us is God, Who also sealed us, and gave the earnest of the Spirit in our hearts.”
79. We were then sealed with the Spirit by God. For as we die in Christ, in order to be born again, so, too, we are sealed with the Spirit, that we may possess His brightness and image and grace, which is undoubtedly our spiritual seal. For although we were visibly sealed in our bodies, we are in truth sealed in our hearts, that the Holy Spirit may portray in us the likeness of the heavenly image.
80. Who, then, can dare to say that the Holy Spirit is separated from the Father and the Son, since through Him we attain to the image and likeness of God, and through Him, as the Apostle Peter says, are partakers of the divine nature? In which there is certainly not the inheritance of carnal succession, but the spiritual connection of the grace of adoption. And in order that we may know that this seal is rather on our hearts than on our bodies, the prophet says: “The light of Your countenance has been impressed upon us, O Lord, You have put gladness in my heart.”
Source: On the Holy Spirit (New Advent)