When we turn to Athanasius’ teaching we are soon made aware of the fact that the Holy Spirit is not cognoscible in himself. In the doctrine of the Spirit we are concerned with the ultimate Being of God before whom the very cherubim veil their faces, for here God the Spirit hides himself not only by the very mode of His Being as Spirit, but by his exaltedness, his greatness and his majesty, that is, by his infinite holiness. Because he is infinitely greater that we can conceive, we think and speak of him in his revelation to us with awe awareness of the weakness of our minds to apprehend him and of the impropriety of the language we use to speak of him (Ad Serapionem 1.17-20). Creatures of course are cognoscible in themselves, and are known in their diversities and in their parts and therefore known through analogies with one another. But it is quite otherwise with the Holy Spirit, who is one and indivisible and cannot be known by our dividing and compounding and comparing, for in his uniqueness he is exalted above everything else and is absolutely incomparable (Ad Serapionem 1.13; cf. C. Arianos 2.18-31 and Basil, De Spiritu Sancto 6.13f.; 9.22f) Thus the very unknowableness of the Spirit by the world (John 14.17, Ad Serapionem 4.1f) means that he belongs to the other side of the chorismos that bounds the existence of the creature, that is, to the side of the Creator himself. Hence the very knowledge of the Spirit carries with it the knowledge that he is in no sense creature, but is himself on the active, creative side of reality which give, imparts, and maintains all other being, and who is known only in and through his own creative activity upon us. To be concerned with the Spirit, to know him, to be acted on by him, is immediately to be concerned with the Being or ousia of God the Creator. That, as I understand it, is the import of the patristic notion of theosis or ‘deification.’
When we ask how Athanasius come to speak like this of the Spirit we find that he does so by moving from the knowledge of the son, and the affirmation of him as homoousios to the Father, to the knowledge of the Spirit, and affirmation of him as homoousios to the Son and the Father.
In examining the movement of his own argument, Athanasius says, ‘It is natural that I should have spoken and written first about the Son of god that form our knowledge of the Son we may be able to have proper knowledge of the Spirit (Ad Ser. 1.31). In meeting the objections of the Tropici to the deity of the Spirit he argued in the first book that we are bound to hold of the Son what he hold of the Spirit of the Son—if the Spirit is a creature, so is the Son. But the deity of the Son has already been established. Hence from the doctrine of the homoousion of the Son he goes on to insist that what we hold of the Son we must also hold of the Spirit of the Son (Ad Ser. 1.22ff.). This does not mean that Athanasius begins with the doctrine of the Son, merely because that already has been established, but that this is the proper procedure because of the propriety of the Spirit to the Son, and because it is only in and through the Son or the Word that God has revealed himself. The Spirit dos not utter himself but the Word and is known only as he enlightens us to understand the Word. The Son is the only logos, the only eidos of Godhead (see C. Arianos 3.15, and Ad Ser. 1.19). Only at that point where in Jesus Christ the Incarnate Word is homoousios with us in our human nature and homoousios with God in his divine Being, is there are real revelation and therefore a knowing of God which really derives from the eternal Being of God as he is in himself. There is no other ‘word’, no other ‘eidos‘ of God, and no other way of knowing God or source of knowledge of him. The Holy Spirit, who is knowable independently in himself, is know through this one self-revelation and self-communication of God in Jesus Christ. Nevertheless, it is only in the Spirit that we may thus know the Son, and know that he is antecedently and eternally in himself in god what he is toward us in revelation and redemption. Only he who is of God (ek Theou) and consubstantial with him can thus impart knowledge of God in himself. It is from the Son that the Spirit shines forth (eklampei, Ad Ser. 1.18), and in the Spirit (en Pneumati) that God is known.