De Synodis 87

VII. "And if any one professing that he believes that there is a Father and a Son, says that the Father is Father of an essence unlike Himself but of similar activity; for speaking profane and novel words against the essence of the Son and nullifying His true divine Sonship, let him be anathema." 20. By confused and involved expressions the heretics very frequently elude the truth and secure the ears of the unwary by the mere sound of common words, such as the titles Father and Son, which they do not truthfully utter to express a natural and genuine community of essence: for they are aware that God is called the Father of all creation, and remember that all the saints are named sons of God. In like manner they declare that the relationship between the Father and the Son resembles that between the Father and the universe, so that the names Father and Son are rather titular than real. For the names are titular if the Persons have a distinct nature of a different essence, since no reality can be attached to the name of father unless it be based on the nature of his offspring. So the Father cannot be called Father of an alien substance unlike His own, for a perfect birth manifests no diversity between itself and the original substance. Therefore we repudiate all the impious assertions that the Father is Father of a Son begotten of Himself and yet not of His own nature. We shall not call God Father for having a creature like Him in might and activity, but for begetting a nature of an essence not unlike or alien to Himself: for a natural birth does not admit of any dissimilarity with the Father's nature. Therefore those are anathema who assert that the Father is Father of a nature unlike Himself, so that something other than God is born of God, and who suppose that the essence of the Father degenerated in begetting the Son. For so far as in them lies they destroy the very birthless and changeless essence of the Father by daring to attribute to Him in the birth of His Only-begotten an alteration and degeneration of His natural essence.