Contra Arianos 1.17-20: God is what He is essentially and from eternity, not accidently. Nothing can be added to God’s essence, nothing can be taken away from it. The Son belongs to this essence from eternity.
Paraphrase: Contra Arianos 1.17: This whole section is very important to the understanding of Athanasius’ conception of God and should be discussed in detail.–If the Son is not from eternity, says Athanasius, then the Trinity is not from eternity, ἀλλά μονὰς μὲν ἦν πρότερον, ἐχ προσθήχης δὲ γέγονεν ὒστερον Τριάς χαὶ προἴόνοτος τοῦ χρόνου χατ’ αὐτοὺς ηὔξησε χαὶ συνέστη τῆς θειλογὶας ἡ γνῶσις . . . χαὶ ποτὲ μὲν ἐλλειτὴς Τριάς, ποτὲ δὲ πλήρης· ἐλλειτὴς μὲν πρὶν γένηται ὁ Υἱός· πλήρης δὲ ὅτε γέγονε. A few sentences later he makes clear why he is opposed to such an addition to God’s being: τὰ γὰρ προστιθέμενα, φανερὸν ὅτι χαὶ ἀφαιπεῖσθαι δύνατια. A similar remark is made in the Ep ad Epict 9: Τριάς ἐστιν, οὐ δεχομένη προσθήχην οὐδὲ ἀφαίρεσιν. Contra Arianos 1.20: For exactly the same reason he opposes any accidental being in the Trinity: πῶς οὐ πολμηρὸν χαὶ δυσσεβὲς εἰπεῖν . . . ὅτι . . . ἐπιστυμβέβηχε, χαὶ δύναται πάλιν μὴ εἶναί ποτε. In Contra Arianos 1.36 he says that if Christ is the Logos ὡς ἐν οὐσία συμβεβηχός . . . ὡς ἐν ἰδυαζούση τινὶ οὐσία συμβεηχέναι τινὰ χάριν χαὶ ἕξιν then the result would be ὤστε χαὶ ἀφαιρεῖσθαι χαὶ προστίθεσθαι αὐτῆ δύνασθαι. In Contra Arianos 3.65 he says of the Arians περὶ . . . τὸν θεὸν φρόνησιν χαὶ βουλὴν χαὶ σοφίαν ὡς ἕξιν συμβαίνουσαν χαὶ ἀποσυμβαίνουσαν ἀνθρωπίνως γίνεσθαι μυθολογοῦσι. It is stressed by the Platonist, too, that there can be no accidens in the divine being. See Albinus: οὐδὲ συμβέβηχέ τι αὐτῷ (sc. τῷ πρώτῳ νῷ), Plotinus, who says of the One: οὔτε τὸ οὔτω οὔτω οὔτε τό ὁπωσοῦν συνέβη, and Proclus: οὐδαμῶς γὰρ προσήχει τῷ ἀεὶ ὄντι τὸ συμβεβηχός. Athanasius statement that, if God is only accidentally what He is, He can cease being what He is, must, of course, be connected with the theory which we have already encountered that what has a beginning must have an end. This theory was, as we saw, important in Middle-Platonism in connection with the creation and preservation of the world.