Sallustius, On the Gods and the Universe 1

Despite its all-embracing title, this is a modest little summary of Greek philosophy, written in the time of Julian the Apostate in the last-ditch fightback against the growth of Christianity. The opening is decidedly uncompromising:

τοὺς περὶ θεῶν ἀκούειν ἐθέλοντας δεῖ μέν ἐκ παίδων ἦχθαι καλῶς, καὶ μὴ ἀνοήτοις συντρέφεσθαι δόξαις· δεῖ δὲ καὶ τὴν φύσιν ἀγαθοὺς εἶναι καὶ ἔμφρονας, ἵνα ὅμοιόν τι ἔχωσι τοῖς λόγοις· δεῖ δὲ αὐτοὺς καὶ τὰς κοινὰς ἐννοίας εἰδέναι.

Those who wish to hear about the gods must have been well educated from childhood, and must not be brought up among foolish suppositions. They must also be naturally good and intelligent, so that they have something in common with the subject matter. And they must also have knowledge of the universal axioms.