Doctrine of the Trinity

The main merit of St. Athanasius lies in his struggle against Arianism. While there was not a single Orthodox bishop left in the Eastern Church, he courageously defended the Orthodox Nicene faith, alone against all, which proclaimed the consubstantiality of the Father and the Son. As discussed above, this non-biblical term had a bad reputation and was condemned at the Council of Antioch in the third century as an expression of the modalistic heresy (if the essence is one, then there is no real difference between the Persons of the Holy Trinity: the Father, the Son, and the Holy Spirit are only different expressions (modes) of one and the same essence).

Athanasius did not go into theological subtleties. Fighting against Arianism for consubstantiality, he defended the very meaning, the very essence of Christianity. The Incarnation, death, and Resurrection of Christ brought salvation to the world. Denying the consubstantiality of Christ with the Father, Arianism threatened to destroy the very essence and foundation of the Christian faith: mankind needs salvation, but salvation is possible only from God, therefore Christ is both man and God, otherwise we are not saved. St. Athanasius firmly knew that only by defending the consubstantial faith could the purity of the Orthodox faith be preserved. He was not embarrassed by the fact that for this purpose it was necessary to introduce an unusual word that most theologians did not like. He considered any philosophical reasoning about the real meaning of this word unnecessary and even dangerous:

We should not inquire why the Word of God is not like ours, because, as has already been said, God is not like us. It is also unseemly to inquire whether the Word is from God, or how He is God's radiance, or how God gives birth and what is the image of God's birth. Whoever dares to undertake such investigations is mad, because he wishes to interpret in words what is ineffably inherent in God's nature and known only to God and His Son. This is the same as searching: where God is, why He is God, and what the Father is like. But just as it is impious and characteristic of those who do not know God, so it is impermissible to dare to make such inquiries about the birth of the Son of God, and to measure God and His Wisdom by one's nature and one's weakness. For this reason, one should not imagine this contrary to the truth; and if anyone is perplexed during these investigations, he should not but believe what is written. It is better for those who are perplexed to be silent and believe, than not to believe because of perplexity.

(Against the Arians, 2, 36)

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: