Concerning the Holy Trinity
We believe, then, in One God, one beginning, having no beginning, uncreate, unbegotten, imperishable and immortal, everlasting, infinite, uncircumscribed, boundless, of infinite power, simple, uncompound, incorporeal, without flux, passionless, unchangeable, unalterable, unseen, the fountain of goodness and justice, the light of the mind, inaccessible; a power known by no measure, measurable only by His own will alone (for all things that He wills He can), creator of all created things, seen or unseen, of all the maintainer and preserver, for all the provider, master and lord and king over all, with an endless and immortal kingdom: having no contrary, filling all, by nothing encompassed, but rather Himself the encompasser and maintainer and original possessor of the universe, occupying all essences intact and extending beyond all things, and being separate from all essence as being super-essential and above all things and absolute God, absolute goodness, and absolute fullness: determining all sovereignties and ranks, being placed above all sovereignty and rank, above essence and life and word and thought: being Himself very light and goodness and life and essence, inasmuch as He does not derive His being from another, that is to say, of those things that exist: but being Himself the fountain of being to all that is, of life to the living, of reason to those that have reason; to all the cause of all good: perceiving all things even before they have become: one essence, one divinity, one power, one will, one energy, one beginning, one authority, one dominion, one sovereignty, made known in three perfect subsistences and adored with one adoration, believed in and ministered to by all rational creation, united without confusion and divided without separation (which indeed transcends thought). (We believe) in Father and Son and Holy Spirit whereinto also we have been baptized. For so our Lord commanded the Apostles to baptize, saying, Baptizing them in the name of the Father, Son, and Holy Spirit.
(We believe) in one Father, the beginning, and cause of all: begotten of no one: without cause or generation, alone subsisting: creator of all: but Father of one only by nature, His Only-begotten Son and our Lord and God and Saviour Jesus Christ, and Producer of the most Holy Spirit. And in one Son of God, the Only-begotten, our Lord, Jesus Christ: begotten of the Father, before all the ages: Light of Light, true God of true God: begotten, not made, consubstantial with the Father, through Whom all things are made: and when we say He was before all the ages we show that His birth is without time or beginning: for the Son of God was not brought into being out of nothing, He that is the effulgence of the glory, the impress of the Father's subsistence, the living wisdom and power, the Word possessing interior subsistence, the essential and perfect and living image of the unseen God. But always He was with the Father and in Him, everlastingly and without beginning begotten of Him. For there never was a time when the Father was and the Son was not, but always the Father and always the Son, Who was begotten of Him, existed together. For He could not have received the name Father apart from the Son: for if He were without the Son, He could not be the Father: and if He thereafter had the Son, thereafter He became the Father, not having been the Father prior to this, and He was changed from that which was not the Father and became the Father. This is the worst form of blasphemy. For we may not speak of God as destitute of natural generative power: and generative power means, the power of producing from one's self, that is to say, from one's own proper essence, that which is like in nature to one's self.
In treating, then, of the generation of the Son, it is an act of impiety to say that time comes into play and that the existence of the Son is of later origin than the Father. For we hold that it is from Him, that is, from the Father's nature, that the Son is generated. And unless we grant that the Son co-existed from the beginning with the Father, by Whom He was begotten, we introduce change into the Father's subsistence, because, not being the Father, He subsequently became the Father. For the creation, even though it originated later, is nevertheless not derived from the essence of God, but is brought into existence out of nothing by His will and power, and change does not touch God's nature. For generation means that the begetter produces out of his essence offspring similar in essence. But creation and making mean that the creator and maker produces from that which is external, and not out of his own essence, a creation of an absolutely dissimilar nature.
Wherefore in God, Who alone is passionless and unalterable, and immutable, and ever so continues, both begetting and creating are passionless. For being by nature passionless and not liable to flux, since He is simple and uncompound, He is not subject to passion or flux either in begetting or in creating, nor has He need of any co-operation. But generation in Him is without beginning and everlasting, being the work of nature and producing out of His own essence, that the Begetter may not undergo change, and that He may not be God first and God last, nor receive any accession: while creation in the case of God, being the work of will, is not co-eternal with God. For it is not natural that that which is brought into existence out of nothing should be co-eternal with what is without beginning and everlasting. There is this difference in fact between man's making and God's. Man can bring nothing into existence out of nothing, but all that he makes requires pre-existing matter for its basis, and he does not create it by will only, but thinks out first what it is to be and pictures it in his mind, and only then fashions it with his hands, undergoing labour and trouble, and often missing the mark and failing to produce to his satisfaction that after which he strives. But God, through the exercise of will alone, has brought all things into existence out of nothing. Now there is the same difference between God and man in begetting and generating. For in God, Who is without time and beginning, passionless, not liable to flux, incorporeal, alone and without end, generation is without time and beginning, passionless and not liable to flux, nor dependent on the union of two: nor has His own incomprehensible generation beginning or end. And it is without beginning because He is immutable: without flux because He is passionless and incorporeal: independent of the union of two again because He is incorporeal but also because He is the one and only God, and stands in need of no co-operation: and without end or cessation because He is without beginning, or time, or end, and ever continues the same. For that which has no beginning has no end: but that which through grace is endless is assuredly not without beginning, as, witness, the angels.
Accordingly the everlasting God generates His own Word which is perfect, without beginning and without end, that God, Whose nature and existence are above time, may not engender in time. But with man clearly it is otherwise, for generation is with him a matter of sex, and destruction and flux and increase and body clothe him round about, and he possesses a nature which is male or female. For the male requires the assistance of the female. But may He Who surpasses all, and transcends all thought and comprehension, be gracious to us.
The holy catholic and apostolic Church, then, teaches the existence at once of a Father: and of His Only-begotten Son, born of Him without time and flux and passion, in a manner incomprehensible and perceived by the God of the universe alone: just as we recognise the existence at once of fire and the light which proceeds from it: for there is not first fire and thereafter light, but they exist together. And just as light is ever the product of fire, and ever is in it and at no time is separate from it, so in like manner also the Son is begotten of the Father and is never in any way separate from Him, but ever is in Him. But whereas the light which is produced from fire without separation, and abides ever in it, has no proper subsistence of its own distinct from that of fire (for it is a natural quality of fire), the Only-begotten Son of God, begotten of the Father without separation and difference and ever abiding in Him, has a proper subsistence of its own distinct from that of the Father.
Source: Exposition of the Faith (New Advent)