Book VIII. Piety is necessary in a Bishop, but he needs also knowledge and dialectical skill in the face of such heresies as were rampant in Hilary's day; for the heretics outdo the orthodox in zeal, and are masters in the art of devising pitfalls for the unwary reasoner (§§ 1-3). He maintains (§ 4) that hitherto he has established his case; and now turns, in § 5, to the Arian interpretation of I and the Father are One, as meaning that They are one in will, not in nature.
The fallacy of this is shown by a comparison of the unity of Christians in Christ (§§ 7-9); a unity which is confessedly one of nature, yet is not more natural than that of Father and Son, of which it is a type (§ 10). And indeed the words, I and the Father are One, are ill-adapted to express a mere harmony of will (§ 11). This gift of unity of nature could not be given, as it is, through the Incarnation and the Eucharist, to Christians, unless the Givers Themselves possessed it; i.e. unless Father and Son were One God (§§ 12-14).
As a matter of fact, we have a perfect union, through the mediation of Christ, with the Father; and it is a unity of nature, a permanent abiding; an assurance to us of the indwelling of Father in Son and Son in Father, and of the fact that Christ is not a creature, one in will with the Father, but a Son, one in nature with Him (§§ 15-18). For, again (§§ 19-21), the Mission of the Holy Ghost is jointly from the Father and the Son; He is called sometimes the Spirit of the Father, sometimes the Spirit of the Son, and this is a further proof of the unity in nature of Father and Son.
Hilary now enquires (§§ 22-25) into the senses in which Scripture speaks of the Holy Spirit. Sometimes this title is given to the Father, sometimes to the Son, in both cases to save us from corporeal conceptions of God. But it is also used, in the strict sense, of the Paraclete, as on the day of Pentecost. Now the Divine Spirit dwells in Christians; but this Spirit, whether styled the Spirit of God, or the Spirit of Christ, or the Spirit of Truth, proceeding from the Father and sent by the Son, is only one Spirit.
Hence the Godhead is One, and the nature of the Persons within that Godhead one also (§§ 26, 27). He next points out (§ 28) that the Arians are inconsistent in worshipping Christ, and yet styling Him a creature; for thus they fall under the curse of the Law, and forfeit the Holy Spirit. Again (§§ 29-34) the powers and graces bestowed by God are described indiscriminately as gifts of one or another Person in the Godhead. The Son, therefore, as a Giver, must be one with the Father, Who is also a Giver, and one with the Spirit.
There is One God and One Lord (§ 35); if we deny that the Son is God, we must also deny that the Father is Lord; which is absurd. They are One God, with one Spirit, but not one Person (§ 36). St. Paul expressly says that Christ is God over all; an expression which must, like all the Apostle's teaching, bear the Catholic sense, and is incompatible with Arianism (§§ 37-39). The supporters of Arianism are thus alien from the faith (§ 40). After a restatement of the truth (§ 41), Hilary proceeds to deduce the Divine nature of the Son from the fact that He has been sealed by the Father (§§ 42-45).
This sealing makes Him the Father's counterpart, Whose Image He thus becomes, though in the form of a servant. If He were thus the Image of God after His Incarnation, how much more before that condescension (§ 46). In § 47 he again denies that this teaching reduces the Father and the Son to one Person; and then (§§ 48-50) works out the sense in which Christ is the Image of God. It means that They are of one nature and of one power, and that the Son is the Firstborn, through Whom all things were created.
But creation and also reconciliation is the joint work of Father and Son (§ 51). Christ could not have stated more explicitly than He has done His unity with the Father; the recognition of this truth is the test of the true Church (§ 52). Heresy is blind to the essential difference between the life-giving Christ and the created universe, which owes its life to Him (§ 53). In Him dwells the whole fullness of the Godhead bodily. The Indweller and the Indwelt are Both Persons, yet are One God; and the whole Godhead dwells in Each (§§ 54-56).
Source: On the Trinity (New Advent)