Origen of Alexandria, Against Celsus 7
Since he repeatedly calls the Christian doctrine "secret," he deserves reproof, because almost the whole world has come to know the Christian proclamation far better than the teachings that appeal to philosophers. After all, what escapes notice? The birth of Jesus from a virgin, his crucifixion, his resurrection believed by many, and the coming judgment that punishes sinners as they deserve while honoring the righteous. Even the mystery of the resurrection, though not understood, is ridiculed by unbelievers. So to claim the doctrine is secret is quite absurd. The idea that there is an inner truth that does not reach the masses is not unique to Christian teaching; philosophers have the same practice. Among them some gave public lectures and others gave private ones. Some heard Pythagoras only through the saying "he himself said," while others were taught in secret doctrines deemed unworthy to reach profane and unpurified ears. And the mysteries celebrated everywhere, in Greece and among the barbarians, though secret, are not slandered. Therefore, without having accurately grasped what is secret about Christianity, he slanders it in vain.
Greek
Εἶτ᾿ ἐπεὶ πολλάκις ὀνομάζει κρύφιον τὸ δόγμα, καὶ ἐν τούτῳ αὐτὸν ἐλεγκτέον, σχεδὸν παντὸς τοῦ κόσμου ἐγνωκότος τὸ κήρυγμα Χριστιανῶν μᾶλλον ἢ τὰ τοῖς φιλοσόφοις ἀρέσκοντα. τίνα γὰρ λανθάνει ἡ ἐκ παρθένου γέννησις Ἰησοῦ καὶ ὁ ἐσταυρωμένος καὶ ἡ παρὰ πολλοῖς πεπιστευμένη ἀνάστασις αὐτοῦ καὶ ἡ καταγγελλομένη κρίσις, κολάζουσα μὲν κατ᾿ ἀξίαν τοὺς ἁμαρτάνοντας γέρως δ᾿ ἀξιοῦσα τοὺς δικαίους; ἀλλὰ καὶ μὴ νοηθὲν τὸ περὶ τῆς ἀναστάσεως μυστήριον θρυλεῖται γελώμενον ὑπὸ τῶν ἀπίστων. ἐπὶ τούτοις οὖν λέγειν κρύφιον εἶναι τὸ δόγμα πάνυ ἐστὶν ἄτοπον· τὸ δ᾿ εἶναί τινα οἷον μετὰ τὰ ἐξωτερικὰ, μὴ εἰς τοὺς πολλοὺς φθάνοντα, οὐ μόνου ἴδιον τοῦ Χριστιανῶν λόγου ἀλλὰ γὰρ καὶ τοῦ φιλοσόφων, παρ᾿ οἷς τινες μὲν ἦσαν ἐξωτερικοὶ λόγοι ἕτεροι δὲ ἐσωτερικοί· καί τινες μὲν ἀκούοντες Πυθαγόρου, ὡς „αὐτὸς ἔφα,“ ἄλλοι δ᾿ ἐν ἀποῤῥήτῳ διδασκόμενοι μενοι τὰ μὴ ἄξια φθάνειν εἰς ἀκοὰς βεβήλους καὶ μηδέπω κεκαθαρμένας. καὶ πάντα δὲ τὰ πανταχοῦ μυστήρια κατὰ τὴν Ἑλλάδα καὶ τὴν βάρβαρον κρύφια ὄντα οὐ διαβέβληται· διόπερ μάτην μηδὲ νοήσας ἀκριβῶς τὸ κρύφιον τοῦ χριστιανισμοῦ διαβάλλει αὐτό.