Letter 807: The priesthood is a sacred trust, not a career.

On the Holy Trinity. To Agathos. Those expressions in sacred Scripture that go beyond the singular number are indicative of the distinction of the hypostases [persons], while those uttered in the singular are indicative of the identity of nature. For the former were spoken so that Sabellius [who denied the distinction of persons] and the Jews might be silenced, and the latter so that Arius and Eunomius [who denied the Son's full divinity] and the pagans might be branded for what they are. For those who, on the one hand, expand the number of units into the Holy Trinity, but, on the other hand, contract it into a single essence, frame their doctrine most correctly and follow the teachings of the heavenly oracles, neither falling away into polytheism through the distinction of nature, nor slipping into Judaism through the positing of a single person.

Περὶ τῆς ἁγίας Τριάδος. Τὰ μὲν τὸν ἑνικὸν ὑπερβαίνοντα ἀριθμὸν ἐν τῇ ἱερᾷ Γραφῇ, τῆς τῶν ὑποστάσεων διαφορᾶς ἐστι παραστατικά· τὰ δ’ ἑνικῶς ἐξεπηνεγμένα, τῆς ταυ- τότητος τῆς φύσεως. Τὰ μὲν γὰρ εἴρηται, ἵνα Σαβέλ- λιος καὶ Ἰουδαῖοι ἐπιστομῶσιν· τὰ δ’ ἵνα Ἄρειος καὶ Εὐνόμιος, καὶ Ἕλληνες στηλιτευθῶσιν. Οἱ γὰρ πλατύνοντες μέν εἰς τὴν ἁγίαν Τριάδα τὸν τῶν ἑνι- τήτων ἀριθμὸν, συστέλλοντες δὲ εἰς μίαν οὐσίαν, ὀρ- θότατα δογματίζουσι, καὶ ταῖς τῶν οὐρανίων χρη- σμῶν ἕπονται διδασκαλίαις, μήτ’ εἰς πολυθεΐαν διὰ τῆς φύσεως διαφορὰν ἐκπίπτοντες, μετ’ εἰς Ἰουδαϊσμὸν, διὰ τὴν τοῦ ἑνὸς προσώπου σύστασιν ὀλισθαίνοντες.