The Son was Begotten of the Father
Thus God the Father, the Founder and Creator of all things, who only knows no beginning, invisible, infinite, immortal, eternal, is one God; to whose greatness, or majesty, or power, I would not say nothing can be preferred, but nothing can be compared; of whom, when He willed it, the Son, the Word, was born, who is not received in the sound of the stricken air, or in the tone of voice forced from the lungs, but is acknowledged in the substance of the power put forth by God, … He then, since He was begotten of the Father, is always in the Father. And I thus say always, that I may show Him not to be unborn, but born. But He who is before all time must be said to have been always in the Father; for no time can be assigned to Him who is before all time. And He is always in the Father, unless the Father be not always Father, only that the Father also precedes Him,—in a certain sense,—since it is necessary—in some degree—that He should be before He is Father. Because it is essential that He who knows no beginning must go before Him who has a beginning [i.e., in a logical sense, in priority]; even as He is the less as knowing that He is in Him, having an origin because He is born, and of like nature with the Father in some measure by His nativity, although He has a beginning in that He is born, inasmuch as He is born of that Father who alone has no beginning.
The Begotten is God from God, Not Another God
He, then, when the Father willed it, proceeded from the Father, and He who was in the Father came forth from the Father; and He who was in the Father because He was of the Father, was subsequently with the Father, because He came forth from the Father,—that is to say, that divine substance whose name is the Word, whereby all things were made, and without whom nothing was made. For all things are after Him, because they are by Him. And reasonably, He is before all things, but after the Father, since all things were made by Him, and He proceeded from Him of whose will all things were made. Assuredly God proceeding from God, causing a person second to the Father as being the Son, but not taking from the Father that characteristic that He is one God.
If He had not been begotten—compared with Him who was not begotten, and as being found equal—they not being begotten, would have reasonably given two Gods, and thus Christ would have been the cause of two Gods. Had He been formed without beginning as the Father, and He Himself the beginning of all things as is the Father, this would have made two beginnings, and consequently would have shown to us two Gods also. Or if He also were not the Son, but the Father begetting from Himself another Son, reasonably, as compared with the Father, and designated as great as He, He would have caused two Fathers, and thus also He would have proved the existence of two Gods. Had He been invisible, as compared with the Invisible, and declared equal, He would have shown forth two Invisibles, and thus also He would have proved them to be two Gods. If incomprehensible, if also whatever other attributes belong to the Father, reasonably we say, He would have given rise to the allegation of two Gods, as these people feign. But now, whatever He is, He is not of Himself, because He is not unborn; but He is of the Father, because He is begotten, whether as being the Word, whether as being the Power, or as being the Wisdom, or as being the Light, or as being the Son; and whatever of these He is, in that He is not from any other source, as we have already said before, than from the Father, owing His origin to His Father.
Moreover, the Son does nothing of His own will, nor does anything of His own determination; nor does He come from Himself, but obeys all His Father’s commands and precepts; so that, although birth proves Him to be a Son, yet obedience even to death declares Him the minister of the will of His Father, of whom He is. Thus making Himself obedient to His Father in all things, although He also is God, yet He shows the one God the Father by His obedience, from whom also He drew His beginning.
He therefore is God, but begotten for this special result, that He should be God. He is also the Lord, but born for this very purpose of the Father, that He might be Lord. He is also an Angel, but He was destined of the Father as an Angel to announce the Great Counsel of God. And His divinity is thus declared, that it may not appear by any dissonance or inequality of divinity to have caused two Gods. For all things being subjected to Him as the Son by the Father, while He Himself, with those things which are subjected to Him, is subjected to His Father, He is indeed proved to be Son of His Father; but He is found to be both Lord and God of all else.
The true and eternal Father is manifested as the one God, from whom alone this power of divinity is sent forth, and also given and directed upon the Son, and is again returned by the communion of substance to the Father. God indeed is shown as the Son, to whom the divinity is beheld to be given and extended. And still, nevertheless, the Father is proved to be one God; while by degrees in reciprocal transfer that majesty and divinity are again returned and reflected as sent by the Son Himself to the Father, who had given them; so that reasonably God the Father is God of all, and the source also of His Son Himself whom He begot as Lord.
— On the Trinity XXXI