The objection has been made, that the words of St. John, “The Spirit is God,” are to be referred to God the Father; since Christ afterwards declares that God is to be worshipped in Spirit and in truth. The answer is, first, that by the word Spirit is sometimes meant spiritual grace; next, it is shown that, if they insist that the Person of the Holy Spirit is signified by the words “in Spirit,” and therefore deny that adoration is due to Him, the argument tells equally against the Son; and since numberless passages prove that He is to be worshipped, we understand from this that the same rule is to be laid down as regards the Spirit. Why are we commanded to fall down before His footstool? Because by this is signified the Lord's Body, and as the Spirit was the Maker of this, it follows that He is to be worshipped, and yet it does not accordingly follow that Mary is to be worshipped. Therefore the worship of the Spirit is not done away with, but His union with the Father is expressed, when it is said that the Father is to be worshipped in Spirit, and this point is supported by similar expressions.
69. But perhaps reference may be made to the fact that in a later passage of the same book, the Lord again said that God is Spirit, but spoke of God the Father. For you have this passage in the Gospel: “The hour now is, when the true worshippers shall worship the Father in Spirit and truth, for such also does the Father seek. God is Spirit, and they that worship Him must worship in Spirit and truth.” By this passage you wish not only to deny the divinity of the Holy Spirit, but also, from God being worshipped in Spirit, deduce a subjection of the Spirit.
70. To which point I will briefly answer that Spirit is often put for the grace of the Spirit, as the Apostle also said: “For the Spirit Himself intercedes for us with groanings which cannot be uttered;” that is, the grace of the Spirit, unless perchance you have been able to hear the groanings of the Holy Spirit. Therefore here too God is worshipped, not in the wickedness of the heart, but in the grace of the Spirit. “For into a malicious soul wisdom does not enter,” because “no one can call Jesus Lord but in the Holy Spirit.” And immediately he adds: “Now there are diversities of gifts.”
71. Now this cannot pertain to the fullness, nor to the dividing of the Spirit; for neither does the mind of man grasp His fullness, nor is He divided into any portions of Himself; but He pours into [the soul] the gift of spiritual grace, in which God is worshipped as He is also worshipped in truth, for no one worships Him except he who drinks in the truth of His Godhead with pious affection. And he certainly does not apprehend Christ as it were personally, nor the Holy Spirit personally.
72. Or if you think that this is said as it were personally of Christ and of the Spirit, then God is worshipped in truth in like manner as He is worshipped in Spirit. There is therefore either a like subjection, which God forbid that you should believe, and the Son is not worshipped; or, which is true, there is a like grace of Unity, and the Spirit is worshipped.
73. Let us then here draw our inferences and put an end to the impious questionings of the Arians. For if they say that the Spirit is therefore not to be worshipped because God is worshipped in Spirit, let them then say that the Truth is not to be worshipped, because God is worshipped in truth. For although there be many truths, since it is written: “Truths are minished from the sons of men;” yet they are given by the Divine Truth, which is Christ, Who says: “I am the Way, the Truth, and the Life.” If therefore they understand the truth in this passage from custom, let them also understand the grace of the Spirit, and there is no stumbling; or if they receive Christ as the Truth, let them deny that He is to be worshipped.
74. But they are refuted by the acts of the pious, and by the course of the Scriptures. For Mary worshipped Christ, and therefore is appointed to be the messenger of the Resurrection to the apostles, loosening the hereditary bond, and the huge offense of womankind. For this the Lord wrought mystically, “that where sin had exceedingly abounded, grace might more exceedingly abound.” And rightly is a woman appointed [as messenger] to men; that she who first had brought the message of sin to man should first bring the message of the grace of the Lord.
75. And the apostles worshipped; and therefore they who bore the testimony of the faith received authority as to the faith. And the angels worshipped, of whom it is written: “And let all His angels worship Him.”
76. But they worship not only His Godhead but also His Footstool, as it is written: “And worship His footstool, for it is holy.” Or if they deny that in Christ the mysteries also of His Incarnation are to be worshipped, in which we observe as it were certain express traces of His Godhead, and certain ways of the Heavenly Word; let them read that even the apostles worshipped Him when He rose again in the glory of His Flesh.
77. Therefore if it do not at all detract from Christ, that God is worshipped in Christ, for Christ too is worshipped; it certainly also detracts nothing from the Spirit that God is worshipped in the Spirit, for the Spirit also is worshipped, as the Apostle has said: “We serve the Spirit of God,” for he who serves worships also, as it is said in an earlier passage: “You shall worship the Lord your God, and Him only shall you serve.”
78. But lest any one should perchance seem to elude the instance we have adduced, let us consider in what manner that which the prophet says, “Worship His Footstool,” appears to refer to the mystery of the divine Incarnation, for we must not estimate the footstool from the custom of men. For neither has God a body, neither is He other than beyond measure, that we should think a footstool was laid down as a support for His feet. And we read that nothing besides God is to be worshipped, for it is written: “You shall worship the Lord your God, and Him only shall you serve.” How, then, should the prophet, brought up under the Law, and instructed in the Law, give a precept against the Law? The question, then, is not unimportant, and so let us more diligently consider what the footstool is. For we read elsewhere: “The heaven is My throne, and the earth the footstool of My feet.” But the earth is not to be worshipped by us, for it is a creature of God.
79. Let us, however, see whether the prophet does not say that that earth is to be worshipped which the Lord Jesus took upon Him in assuming flesh. And so, by footstool is understood earth, but by the earth the Flesh of Christ, which we this day also adore in the mysteries, and which the apostles, as we said above, adored in the Lord Jesus; for Christ is not divided but is one; nor, when He is adored as the Son of God, is He denied to have been born of the Virgin. Since, then, the mystery of the Incarnation is to be adored, and the Incarnation is the work of the Spirit, as it is written, “The Holy Spirit shall come upon you, and the power of the Most High shall overshadow you, and that Holy Thing Which shall be born of you shall be called the Son of God,” without doubt the Holy Spirit also is to be adored, since He Who according to the flesh was born of the Holy Spirit is adored.
80. And let no one divert this to the Virgin Mary; Mary was the temple of God, not the God of the temple. And therefore He alone is to be worshipped Who was working in His temple.
Source: On the Holy Spirit (New Advent)