Diodorus of Tarsus (d. 390)
The reference, as indicated by Metzger, is Cranmer's
Catanae for
Romans, p. 162 lines 25-27. It's from the
Fragmenta in epistulam ad Romanos (Fragments from his commentary on Romans)
Ἀναγκαίως τῷ παρέποντι ὅρκῳ πιστοῦται τὸν λόγον· προειπὼν γὰρ ὅτι οὐκ ἔστι τι ὃ δυνήσεται αὐτὸν χωρίσαι τῆς ἀγάπης τοῦ Χριστοῦ, ἐπάγει ὅτι μόνον τῆς συγγενείας αὐτοῦ ἡττᾶται. φησὶν οὖν ὅτι ὑπὲρ τῶν Ἰουδαίων ἡδέως ἂν καὶ σῶμα καὶ ψυχὴν προήκατο, ἵνα ὧν ἐκπεπτώκασιν, τὰ αὐτὰ πάλιν ἀνακαλέσωνται, πιστεύσαντες εἰς Χριστόν. τί γὰρ τῶν καλῶν οὐκ Ἰουδαίων ἦν ἐφ' οἷς νῦν τὰ ἔθνη σεμνύνεται; ἡ υἱοθεσία, ἡ δόξα, αἱ διαθῆκαι, αἱ ἐπαγγελίαι, ἡ λατρεία, οἱ θεοφιλεῖς πατέρες, καὶ τὸ μέγιστον· ἐξ ὧν ὁ Χριστὸς τὸ κατὰ σάρκα. ἐξ αὐτῶν, φησίν, ὁ Χριστός· θεὸς δὲ οὐ μόνον αὐτῶν, ἀλλὰ κοινῇ ἐπὶ πάντων ἐστὶ θεός. τούτων οὖν πάντων ἐκπεπτώκασιν Ἰουδαῖοι, ἀλλ' ἵνα ταῦτα ἀνακτήσωνται, φησίν, ἡδέως ἂν ἀνάθεμα ὑπὲρ αὐτῶν ἐγενόμην ἀπὸ τοῦ Χριστοῦ. τὸ δὲ ἀνάθεμα τὸν ἀλλότριον δηλοῖ, ἐπεὶ τὸ ἀνατεθὲν τοῦ ἀνατιθέντος ἀλλότριον. ἀναγκαίως δὲ ὅσην ἔχει περὶ αὐτοὺς διάθεσιν δείκνυσιν, ἵνα μή τις τοὺς κατ' αὐτῶν ἐλέγχους πρὸς ἀπέχθειαν λέγεσθαι ὑπολάβοι.
I'll let you translate this one on your own. Should be a piece of cake for you, since you're always correcting and insulting the Greek of others. I don't personally see how θεὸς δὲ οὐ μόνον αὐτῶν, ἀλλὰ κοινῇ ἐπὶ πάντων ἐστὶ θεός refers, specifically, to God the Father. Maybe you see it? It seems to be a continuation of the thought ἐξ αὐτῶν, φησίν, ὁ Χριστός. Cranmer's
Catenae also does not include the following words that close out the chapter which seems important to the overall context:
ἐπειδὴ δήποτε ἐπαιδεύθη τὰ ἔθνη διὰ τῆς τῶν Ἰουδαίων ἐπιμιξίας τὸν ἐπὶ πάντων γνωρίζειν θεόν, τηνικαῦτα καὶ ὁ σωτὴρ ἡμῶν Ἰησοῦς Χριστὸς ἐν σαρκὶ ἐπιδημήσας τῷ κόσμῳ, τοὺς ἑαυτοῦ μαθητὰς ἀποστέλλει κήρυκας αὐτοῦ τῆς γνώσεως. διαλανθανούσης μὲν ἐν χρόνοις, κηρυχθησομένης δὲ ἐν ἐσχάτοις καιροῖς, οἳ καὶ πανταχόσε φοιτῶντες κατήγγελλον τὸν Χριστὸν θεὸν ἐκ θεοῦ ἀνάρχως καὶ ἀχρόνως γεγεννημένον, μαρτυροῦντες ἅμα καὶ τῷ ἁγίῳ καὶ φωτιστικῷ πνεύματι, ὅτι τὴν ὁμοούσιον τριάδα συμπληροῖ ἐκ πατρὸς δι' υἱοῦ εἰς ἀνθρώπους χορηγούμενον. τῆς οἰκονομίας οὖν αὐτοῦ τὴν σοφίαν καταπλαγεὶς τὴν ὀφειλομένην ἀναπέμπει δόξαν τῷ θεῷ.
If you get stuck translating, let me know.