Texts Explained; Ninthly, John 10:30; 17:11, etc. Arianexplanation, that the Son is one with the Father in will and judgment; but so are all good men, nay things inanimate; contrast of the Son. Oneness between Them is in nature, because oneness in operation. Angels not objects of prayer, because they do not work together with God, but the Son; texts quoted. Seeing an Angel, is not seeing God. Ariansin fact hold two Gods, and tend to Gentile polytheism. Arianexplanation that the Father and Son are one as we are one with Christ, is put aside by the Regula Fidei, and shown invalid by the usage of Scripture in illustrations; the true force of the comparison; force of the terms used. Force of 'in us;' force of 'as;' confirmed by St. John. In what sense we are 'in God' and His 'sons.'
24 We then, by way of giving a rude view of the expressions in this passage, have been led into many words, but blessed John will show from his Epistle the sense of the words, concisely and much more perfectly than we can. And he will both disprove the interpretation of these irreligious men, and will teach how we become in God and God in us; and how again we become One in Him, and how far the Son differs in nature from us, and will stop the Arians from any longer thinking that they shall be as the Son, lest they hear it said to them, 'You are a man and not God,' and 'Stretch not yourself, being poor, beside a rich man. ' John then thus writes; 'Hereby know we that we dwell in Him and He in us, because He has given us of His Spirit.' Therefore because of the grace of the Spirit which has been given to us, in Him we come to be, and He in us; and since it is the Spirit of God, therefore through His becoming in us, reasonably are we, as having the Spirit, considered to be in God, and thus is God in us. Not then as the Son in the Father, so also we become in the Father; for the Son does not merely partake the Spirit, that therefore He too may be in the Father; nor does He receive the Spirit, but rather He supplies It Himself to all; and the Spirit does not unite the Word to the Father, but rather the Spirit receives from the Word. And the Son is in the Father, as His own Word and Radiance; but we, apart from the Spirit, are strange and distant from God, and by the participation of the Spirit we are knit into the Godhead; so that our being in the Father is not ours, but is the Spirit's which is in us and abides in us, while by the true confession we preserve it in us, John again saying, 'Whosoever shall confess that Jesus is the Son of God, God dwells in him and he in God.' What then is our likeness and equality to the Son? Rather, are not the Arians confuted on every side? And especially by John, that the Son is in the Father in one way, and we become in Him in another, and that neither we shall ever be as He, nor is the Word as we; except they shall dare, as commonly, so now to say, that the Son also by participation of the Spirit and by improvement of conduct came to be Himself also in the Father. But here again is an excess of irreligion, even in admitting the thought. For He, as has been said, gives to the Spirit, and whatever the Spirit has, He has from the Word.
25. The Saviour, then, saying of us, 'As Thou, Father, art in Me, and I in You, that they too may be one in Us,' does not signify that we were to have identity with Him; for this was shown from the instance of Jonah; but it is a request to the Father, as John has written, that the Spirit should be vouchsafed through Him to those who believe, through whom we are found to be in God, and in this respect to be conjoined in Him. For since the Word is in the Father, and the Spirit is given from the Word, He wills that we should receive the Spirit, that, when we receive It, thus having the Spirit of the Word which is in the Father, we too may be found on account of the Spirit to become One in the Word, and through Him in the Father. And if He say, 'as we,' this again is only a request that such grace of the Spirit as is given to the disciples may be without failure or revocation. For what the Word has by nature, as I said, in the Father, that He wishes to be given to us through the Spirit irrevocably; which the Apostle knowing, said, 'Who shall separate us from the love of Christ.' for 'the gifts of God?' and 'grace of His calling are without repentance. ' It is the Spirit then which is in God, and not we viewed in our own selves; and as we are sons and gods because of the Word in us, so we shall be in the Son and in the Father, and we shall be accounted to have become one in Son and in Father, because that that Spirit is in us, which is in the Word which is in the Father. When then a man falls from the Spirit for any wickedness, if he repent upon his fall, the grace remains irrevocably to such as are willing; otherwise he who has fallen is no longer in God (because that Holy Spirit and Paraclete which is in God has deserted him), but the sinner shall be in him to whom he has subjected himself, as took place in Saul's instance; for the Spirit of God departed from him and an evil spirit was afflicting him. God's enemies hearing this ought to be henceforth abashed, and no longer to feign themselves equal to God. But they neither understand (for 'the irreligious,' he says, 'does not understand knowledge') nor endure religious words, but find them heavy even to hear.
Source: Four Discourses Against the Arians (New Advent)