Hilary of Poitiers De Trin., 6:34 also 4.6,14

6.34. By this the holy Apostles did not understand that He had gone forth, in the sense of having been sent, from God. For they had often heard Him confess, in His earlier discourses, that He was sent; but what they hear now is the express statement that He had gone forth from God. This opens their eyes to perceive from His works His Divine nature. The fact that He had gone forth from God makes clear to them His true Divinity, and so they say, Now therefore we are sure that Thou knowest all things, and needest not that any man should ask Thee; by this we believe that Thou wentest forth from God. The reason why they believe that He went forth from God is that He both can, and does, perform the works of God. Their perfect assurance of His Divine nature is the result of their knowledge, not that He is come from God, but that He did go forth from God. Accordingly we find that it is this truth, now heard for the first time, which clenches their faith. The Lord had made two statements; I went forth from God, and I am come from the Father into this world. One of these, I am come from the Father into this world, they had often heard, and it awakens no surprise. But their reply makes it manifest that they now believe and understand the other, that is, I went forth from God. Their answer, By this we believe that Thou wentest forth from God, is a response to it, and to it only; they do not add, ‘And art come from the Father into this world.’ The one statement is welcomed with a declaration of faith; the other is passed over in silence. The confession was wrung from them by the sudden presentation of a new truth, which convinced their reason and constrained them to avow their certainty. They knew already that He, like God, could do all things; but His birth, which accounted for that omnipotence, had not been revealed. They knew that He had been sent from God, but they knew not that He had gone forth from God. Now at last, taught by this utterance to understand the ineffable and perfect birth of the Son, they confess that He had spoken to them without a proverb.

See Also 4.6.

What foolish and godless fears! What impious anxiety on God’s behalf! The meaning which they profess to detect in the word homoousion, and in the assertion of the eternity of the Son, is detested, rejected, denounced by the Church. She confesses one God front Whom are all things; she confesses one Jesus Christ our Lord, through whom are all things; One from Whom, One through Whom; One the Source of all, One the Agent through Whom all were created. In the One from Whom are all things she recognises the Majesty which has no beginning, and in the One through Whom are all things she recognises a might coequal with His Source; for Both are jointly supreme in the work of creation and in rule over created things. In the Spirit she recognises God as Spirit, impossible and indivisible, for she has learnt from the Lord that Spirit has neither flesh nor bones; a warning to save her from supposing that God, being Spirit, could be burdened with bodily suffering and loss. She recognises one God, unborn from everlasting; she recognises also one Only-begotten Son of God. She confesses the Father eternal and without beginning; she confesses also that the Son’s beginning is from eternity. Not that He has no beginning, but that He is Son of the Father Who has none; not that He is self-originated, but that He is from Him Who is unbegotten from everlasting; born from eternity, receiving, that is, His birth from the eternity of the Father. Thus our faith is free from the guesswork of heretical perversity; it is expressed in fixed and published terms, though as yet no reasoned defence of our confession has been put forth. Still, lest any suspicion should linger around the sense in which the Fathers have used the word homoousion and round our confession of the eternity of the Son, I have set down the proofs whereby we may be assured that the Son abides ever in that substance wherein He was begotten from the Father, and that the birth of His Son has not diminished ought of that Substance wherein the Father was abiding; that holy men, inspired by the teaching of God, when they said that the Son is homoousios with the Father pointed to no such flaws or defects as I have mentioned. My purpose has been to counteract the impression that this ousia, this assertion that He is homoousios with the Father, is a negation of the nativity of the Only-begotten Son.

and 4.14.

Such is their error, such their pestilent teaching; to support it they borrow the words of Scripture, perverting its meaning and using the ignorance of men as their opportunity of gaining credence for their lies. Yet it is certainly by these same words of God that we must come to understand the things of God. For human feebleness cannot by any strength of its own attain to the knowledge of heavenly things; the faculties which deal with bodily matters can form no notion of the unseen world. Neither our created bodily substance, nor the reason given by God for the purposes of ordinary life, is capable of ascertaining and pronouncing upon the nature and work of God. Our wits cannot rise to the level of heavenly knowledge, our powers of perception lack the strength to apprehend that limitless might. We must believe God’s word concerning Himself, and humbly accept such insight as He vouchsafes to give. We must make our choice between rejecting His witness, as the heathen do, or else believing in Him as He is, and this in the only possible way, by thinking of Him in the aspect in which He presents Himself to us. Therefore let private judgment cease; let human reason refrain from passing barriers divinely set. In this spirit we eschew all blasphemous and reckless assertion concerning God, and cleave to the very letter of revelation. Each point in our enquiry shall be considered in the light of His instruction, Who is our theme; there shall be no stringing together of isolated phrases whose context is suppressed, to trick and misinform the unpractised listener. The meaning of words shall be ascertained by considering the circumstances under which they were spoken words must be explained by circumstances not circumstances forced into conformity will words. We, at any rate, will treat our subject completely; we will state both the circumstances under which words were spoken, and the true purport of the words. Each point shall be considered in orderly sequence.