Texts Explained; Seventhly, John 14:10 Introduction. The doctrine of the coinherence. The Father and the Son Each whole and perfect God. They are in Each Other, because their Essence is One and the Same. They are Each Perfect and have One Essence, because the Second Person is the Son of the First. Asterius's evasive explanation of the text under review; refuted. Since the Son has all that the Father has, He is His Image; and the Father is the One God, because the Son is in the Father
1 The Ario-maniacs, as it appears, having once made up their minds to transgress and revolt from the Truth, are strenuous in appropriating the words of Scripture, 'When the impious comes into a depth of evils, he despises;' for refutation does not stop them, nor perplexity abash them; but, as having 'a whore's forehead,' they 'refuse to be ashamed ' before all men in their irreligion. For whereas the passages which they alleged, 'The Lord created me,' and 'Made better than the Angels,' and 'First-born,' and 'Faithful to Him that made Him ' have a right sense, and inculcate religiousness towards Christ, so it is that these men still, as if bedewed with the serpent's poison, not seeing what they ought to see, nor understanding what they read, as if in vomit from the depth of their irreligious heart, have next proceeded to disparage our Lord's words, 'I in the Father and the Father in Me;' saying, 'How can the One be contained in the Other and the Other in the One?' or 'How at all can the Father who is the greater be contained in the Son who is the less?' or 'What wonder, if the Son is in the Father,' considering it is written even of us, 'In Him we live and move and have our being?' And this state of mind is consistent with their perverseness, who think God to be material, and understand not what is 'True Father?' and 'True Son,' nor 'Light Invisible' and 'Eternal,' and Its 'Radiance Invisible,' nor 'Invisible Subsistence,' and 'Immaterial Expression' and 'Immaterial Image.' For did they know, they would not dishonour and ridicule the Lord of glory, nor interpreting things immaterial after a material manner, pervert good words. It were sufficient indeed, on hearing only words which are the Lord's, at once to believe, since the faith of simplicity is better than an elaborate process of persuasion; but since they have endeavoured to profane even this passage to their own heresy, it becomes necessary to expose their perverseness and to show the mind of the truth, at least for the security of the faithful. For when it is said, 'I in the Father and the Father in Me,' They are not therefore, as these suppose, discharged into Each Other, filling the One the Other, as in the case of empty vessels, so that the Son fills the emptiness of the Father and the Father that of the Son, and Each of Them by Himself is not complete and perfect (for this is proper to bodies, and therefore the mere assertion of it is full of irreligion), for the Father is full and perfect, and the Son is the Fulness of Godhead. Nor again, as God, by coming into the Saints, strengthens them, thus is He also in the Son. For He is Himself the Father's Power and Wisdom, and by partaking of Him things originate are sanctified in the Spirit; but the Son Himself is not Son by participation, but is the Father's own Offspring. Nor again is the Son in the Father, in the sense of the passage, 'In Him we live and move and have our being;' for, He as being from the Fount of the Father is the Life, in which all things are both quickened and consist; for the Life does not live in life, else it would not be Life, but rather He gives life to all things.
2. But now let us see what Asterius the Sophist says, the retained pleader for the heresy. In imitation then of the Jews so far, he writes as follows; 'It is very plain that He has said, that He is in the Father and the Father again in Him, for this reason, that neither the word on which He was discoursing is, as He says, His own, but the Father's, nor the works belong to Him, but to the Father who gave Him the power.' Now this, if uttered at random by a little child, had been excused from his age; but when one who bears the title of Sophist, and professes universal knowledge, is the writer, what a serious condemnation does he deserve! And does he not show himself a stranger to the Apostle, as being puffed up with persuasive words of wisdom, and thinking thereby to succeed in deceiving, not understanding himself what he says nor whereof he affirms? For what the Son has said as proper and suitable to a Son only, who is Word and Wisdom and Image of the Father's Essence, that he levels to all the creatures, and makes common to the Son and to them; and he says, lawless man, that the Power of the Father receives power, that from this his irreligion it may follow to say that in a son the Son was made a son, and the Word received a word's authority; and, far from granting that He spoke this as a Son, He ranks Him with all things made as having learned it as they have. For if the Son said, 'I am in the Father and the Father in Me,' because His discourses were not His own words but the Father's, and so of His works, then—since David says, 'I will hear what the Lord God shall say in me,' and again Solomon, 'My words are spoken by God,' and since Moses was minister of words which were from God, and each of the Prophets spoke not what was his own but what was from God, 'Thus says the Lord,' and since the works of the Saints, as they professed, were not their own but God's who gave the power, Elijah for instance and Elisha invoking God that He Himself would raise the dead, and Elisha saying to Naaman, on cleansing him from the leprosy, 'that you may know that there is a God in Israel,' and Samuel too in the days of the harvest praying to God to grant rain, and the Apostles saying that not in their own power they did miracles but in the Lord's grace— it is plain that, according to Asterius such a statement must be common to all, so that each of them is able to say, 'I in the Father and the Father in me;' and as a consequence that He is no longer one Son of God and Word and Wisdom, but, as others, is only one out of many.
3. But if the Lord said this, His words would not rightly have been, 'I in the Father and the Father in Me,' but rather, 'I too am in the Father, and the Father is in Me too,' that He may have nothing of His own and by prerogative, relatively to the Father, as a Son, but the same grace in common with all. But it is not so, as they think; for not understanding that He is genuine Son from the Father, they belie Him who is such, whom alone it befits to say, 'I in the Father and the Father in Me.' For the Son is in the Father, as it is allowed us to know, because the whole Being of the Son is proper to the Father's essence, as radiance from light, and stream from fountain; so that whoso sees the Son, sees what is proper to the Father, and knows that the Son's Being, because from the Father, is therefore in the Father. For the Father is in the Son, since the Son is what is from the Father and proper to Him, as in the radiance the sun, and in the word the thought, and in the stream the fountain: for whoso thus contemplates the Son, contemplates what is proper to the Father's Essence, and knows that the Father is in the Son. For whereas the Form and Godhead of the Father is the Being of the Son, it follows that the Son is in the Father and the Father in the Son.
Source: Four Discourses Against the Arians (New Advent)