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 from 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, impassible and indivisible, for she has learned 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.
Source: On the Trinity (New Advent)