The Holy Creed of Sirmium, from the council of Sirmium in 357:

Since there appeared to be some misunderstanding respecting the faith, all points have been carefully investigated and discussed at Sirmium in the presence of our most reverend brothers and fellow bishops, Valens, Ursacius and Germinius.

It is evident that there is one God, the Father Almighty, according as it is believed throughout the whole world; and His only Son Jesus Christ our Saviour, begotten of Him before the ages. But we cannot and ought not to say that there are two Gods, for the Lord Himself said, I will go unto My Father and your Father, unto My God and your God. John 20:17 So there is one God over all, as the Apostle has taught us, Is He God of the Jews only? Is He not also of the Gentiles? Yes, of the Gentiles also: seeing it is one God, which shall justify the circumcision by faith, and the uncircumcision through faith. And in all other things they agreed thereto, nor would they allow any difference.

But since some or many persons were disturbed by questions concerning substance, called in Greek οὐσία, that is, to make it understood more exactly, as to ὁμοούσιον, or what is called ὁμοιούσιον, there ought to be no mention made of these at all. Nor ought any exposition to be made of them for the reason and consideration that they are not contained in the divine Scriptures, and that they are above man’s understanding, nor can any man declare the birth of the Son, of whom it is written, Who shall declare His generation Isaiah 53:8 ? For it is plain that only the Father knows how He begot the Son, and the Son how He was begotten of the Father. There is no question that the Father is greater. No one can doubt that the Father is greater than the Son in honour, dignity, splendour, majesty, and in the very name of Father, the Son Himself testifying, He that sent Me is greater than I. John 14:28 And no one is ignorant that it is Catholic doctrine that there are two Persons of Father and Son; and that the Father is greater, and that the Son is subordinated to the Father, together with all things which the Father has subordinated to Him, and that the Father has no beginning and is invisible, immortal and impassible, but that the Son has been begotten of the Father, God of God, Light of Light, and that the generation of this Son, as is aforesaid, no one knows but His Father. And that the Son of God Himself, our Lord and God, as we read, took flesh, that is, a body, that is, man of the womb of the Virgin Mary, of the Angel announced. And as all the Scriptures teach, and especially the doctor of the Gentiles himself, He took of Mary the Virgin, man, through whom He suffered. And the whole faith is summed up and secured in this, that the Trinity must always be preserved, as we read in the Gospel, Go and baptize all nations in the Name of the Father, and of the Son, and of the Holy Ghost. Matthew 28:19 Complete and perfect is the number of the Trinity; but the Paraclete, the Holy Ghost, sent forth through the Son, came according to the promise, that He might teach and sanctify the Apostles and all believers..