The Father's twofold begetting of the Son
Cyril is explaining how the Father's words in Psalm 2:7 reveal both the Son's eternal existence and his birth in the flesh from the Virgin.
Cyril says: You see the confession at the very start, that is, the declaration. For the Father has set down the words "You are" in order to show the Son's eternal being. He did not become Son in time; he always was this—that is, Son. Then the Father brought in the words "Today I have begotten you" (Psalm 2:7) in order to show his newest birth according to the flesh, which he willingly underwent through the holy Virgin. This is why she has been named Mother of God. For she gave birth not to a mere human being simply joined to God by an external connection, but rather to the very Word from God the Father, who put on human poverty and became a human being like us.
Greek
Ὁρᾷς ἐν ἀρχῇ τὴν ὁμολογίαν, ἤγουν τὴν ἀνάρρησιν· τὸ μὲν γὰρ εἶ τέθεικεν,
ἵνα δείξῃ τὸ προαιώνιον· οὐ γὰρ ἐν χρόνῳ γέγονεν, ἀλλ' ἦν ἀεὶ τοῦτο, τουτέστιν
υἱός. ἐπήνεγκε δὲ τό Σήμερον ἐγέννησά σε, ἵνα τὴν νεωτάτην αὐτοῦ καὶ κατὰ σάρκα
δείξῃ γέννησιν, ἣν ὑπέμεινεν ἑκὼν διὰ τῆς ἁγίας Παρθένου. ταύτῃ τοι καὶ Θεοτόκος
ὠνόμασται· τέτοκε γὰρ οὐκ ἄνθρωπον ἁπλῶς κατά γε τὴν ἔξωθεν σχέσιν
συναφθέντα τῷ Θεῷ, αὐτὸν δὲ μᾶλλον τὸν ἐκ Θεοῦ Πατρὸς Λόγον τὴν ἀνθρώπειαν
ὑποδύντα πτωχείαν καὶ γενόμενον καθ' ἡμᾶς ἄνθρωπον.