Athanasius speaks of the Holy Spirit in his letters to Serapion. His reasoning follows the same logic as his teaching about Christ. There is one God, the one-in-essence Trinity: the Father, the Son, and the Spirit. Like the Son, the Holy Spirit is also God, otherwise we are not saved:

... Of the Holy Spirit, God says that He is not just a Spirit, but His Spirit, and that by Him our spirit is renewed, just as the Psalmist says in the one hundred and third Psalm: "If you take away their spirit, they die, and return to their dust. Thou shalt send Thy Spirit, and they shall be built, and Thou dost renew the face of the earth" (Psalm 103:29,30). If we are renewed by the Spirit of God, then it is not the Holy Spirit, but our spirit that is called created here. And if, on the basis that everything is brought into being by the Word, you admirably reason that the Son is not a creature, then is it not blasphemous to call the creature of the Spirit, by which the Father through the Word brings all things to perfection and renews them?

(Serapion, Epistle 1:16,17)

Through the Holy Spirit, man's communion with God and deification take place. At the same time, the Spirit belongs to the Son, being His "own" (idios) Spirit:

Thus, according to the grace of the Spirit given to us, we are in him, and He is in us. And since the Spirit that is in us is God's, then we, having the Spirit in us, are justly considered to be in God, and thus God is in us. Therefore, we do not dwell in the Father as the Son does. The Son does not become a partaker of the Spirit, so that through this He may be in the Father. He does not receive the Spirit, but rather He Himself gives Him to all, and it is not the Spirit who unites the Son with the Father, but rather the Spirit receives from the Word. And the Son is in the Father, as His own Word and radiance. And without the Spirit we are alien to God and far from Him, but through the communion of the Spirit we are united with the Divinity.

(Against, Arian. 3:24)

The phrase "rather He Himself gives Him to all" has often been misinterpreted as an argument in favor of the Filioque: as if in this case St. Athanasius asserts that the Holy Spirit proceeds from the Son. Such an understanding overlooks the context of the statement, which does not discuss the eternal procession of the Spirit at all. Athanasius was alien to the philosophical approach to the existence of God, which gave rise to the doctrine of the Filioque, and in his writings he never speaks of the Holy Trinity in abstract terms. Athanasius' thought is always focused on the idea of saving man from slavery to death and corruption. Salvation was brought into the world by Christ, and He also sends us the Holy Spirit, through whom we partake of the divine life.

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