Cyril is here in full accord with Athanasius:  in his Creed he found “Son of God,” and in his exposition he states that the Father is “by nature and in truth Father of One only, the Only-begotten Son Cat. vii. § 5.:”  “One they are because of the dignity pertaining to the Godhead, since God begat God Ib. xi. § 16.:”  “The Son then is Very God, having the Father in Himself, not changed into the Father Ib. § 17..”  When he says that the Son is in all things like (ὅμοιος ἐν πᾶσιν) to Him who begat Him; begotten Life of Life, and Light of Light, Power of Power, God of God, and the characteristics of the Godhead are unchangeable (ἀπαράλλακτοι ) in the Son Ib. § 18.,” he is using in all good faith the very words of the orthodox Bishops at Nicæa, “ὅμοιόν τε καὶ ἀπαράλλακτον αὐτὸν κατὰ πὰντα τῷ Πατρί Athan. De Decretis, c. 20..”