Plato knew the truth, yet was without excuse
Let the wise men of old, then, make things known to those able to understand them; and let Plato, the son of Ariston, indicate in one of his letters what concerns the first good, and say that the first good can by no means be “spoken”, but that it arises “from long association” and appears “suddenly, like a light kindled from a leaping fire, in the soul” (Plato, Seventh Letter 341c–d). When we hear these words we too agree with them as well said, “for God showed them” (Romans 1:19) these things and whatever else has been well said.
This is why we say that even those who have grasped the truth about God, but have not practised the worship of God that is worthy of the truth about him, are liable to the punishments of sinners. For Paul says of such people in so many words: “The wrath of God is revealed from heaven against all ungodliness and unrighteousness of men who hold the truth in unrighteousness, because what can be known of God is plain among them, for God showed it to them. For his invisible things, from the creation of the world, are clearly seen, being understood through the things that are made, both his eternal power and divinity, so that they are without excuse; because, although they knew God, they did not glorify him as God or give thanks, but became futile in their reasonings, and their senseless heart was darkened. Claiming to be wise, they became fools, and exchanged the glory of the incorruptible God for the likeness of an image of corruptible man, and of birds and four-footed beasts and creeping things” (Romans 1:18–23).
So those people do indeed “hold the truth”, as our own word also bears witness, who think that the first good “can by no means be spoken”, and say that “from long association with the matter itself, and from living with it, suddenly, like a light kindled from a leaping fire, it comes to be in the soul and from then on nourishes itself” (Plato, Seventh Letter 341c–d).
Greek
Παλαιοὶ τοίνυν ἄνδρες καὶ σοφοὶ δηλούσθωσαν τοῖς ἐπίστασθαι δυναμένοις, καὶ δὴ καὶ Πλάτων ὁ τοῦ Ἀρίστωνος τὰ περὶ τοῦ πρώτου ἀγαθοῦ διασημαινέτω ἔν τινι τῶν ἐπιστολῶν καὶ φασκέτω μηδαμῶς εἶναι „ῥητὸν“ τὸ πρῶτον ἀγαθὸν, „ἀλλ᾿ ἐκ πολλῆς συνουσίας“ ἐγγινόμενον καὶ „ἐξαίφνης οἷον ἀπὸ πυρὸς πηδήσαντος ἐξαφθὲν φῶς ἐν τῇ ψυχῇ“· ὧν καὶ ἡμεῖς ἀκούσαντες συγκατατιθέμεθα ὡς καλῶς λεγομένοις, „ὁ θεὸς γὰρ αὐτοῖς“ ταῦτα καὶ ὅσα καλῶς λέλεκται „ἐφανέρωσε.“ διὰ τοῦτο δὲ καὶ τοὺς τὰ ἀληθῆ περὶ θεοῦ ὑπολαβόντας καὶ μὴ τὴν ἀξίαν τῆς περὶ αὐτοῦ ἀληθείας θεοσέβειαν ἀσκήσαντάς φαμεν ὑποκεῖσθαι ταῖς τῶν ἁμαρτανόντων κολάσεσιν. αὐταῖς γὰρ λέξεσί φησι περὶ τῶν τοιούτων ὁ Παῦλος ὅτι „ἀποκαλύπτεται ὀργὴ θεοῦ ἀπ᾿ οὐρανοῦ ἐπὶ πᾶσαν ἀσέβειαν καὶ ἀδικίαν ἀνθρώπων τῶν τὴν ἀλήθειαν ἐν ἀδικίᾳ κατεχόντων, διότι τὸ γνωστὸν τοῦ θεοῦ φανερόν ἐστιν ἐν αὐτοῖς· ὁ θεὸς γὰρ αὐτοῖς ἐφανέρωσε. τὰ γὰρ ἀόρατα αὐτοῦ ἀπὸ κτίσεως κόσμου τοῖς ποιήμασι νοούμενα καθορᾶται, ἥ τε ἀίδιος αὐτοῦ δύναμις καὶ θειότης, εἰς τὸ εἶναι αὐτοὺς ἀναπολογήτους, διότι γνόντες τὸν θεὸν οὐχ ὡς θεὸν ἐδόξασαν ἢ ηὐχαρίστησαν, ἀλλ᾿ ἐματαιώθησαν ἐν τοῖς διαλογισμοῖς αὐτῶν, καὶ ἐσκοτίσθη ἡ ἀσύνετος αὐτῶν καρδία. φάσκοντες εἶναι σοφοὶ ἐμωράνθησαν, καὶ ἤλλαξαν τὴν δόξαν τοῦ ἀφθάρτου θεοῦ ἐν ὁμοιώματι εἰκόνος φθαρτοῦ ἀνθρώπου καὶ πετεινῶν καὶ τετραπόδων καὶ ἑρπετῶν.“ καὶ „ἀλήθειαν“ γοῦν κατέχουσιν, ὡς καὶ ὁ ἡμέτερος μαρτυρεῖ λόγος, οἱ φρονοῦντες ὅτι „ῥητὸν οὐδαμῶς ἐστι“ τὸ πρῶτον ἀγαθὸν καὶ λέγοντες ὡς „ἐκ πολλῆς συνουσίας γινομένης περὶ τὸ πρᾶγμα αὐτὸ καὶ τοῦ συζῆν ἐξαίφνης οἷον ἀπὸ πυρὸς πηδήσαντος ἐξαφθὲν φῶς ἐν τῇ ψυχῇ γενόμενον αὐτὸ ἑαυτὸ ἤδη τρέφει.“