Diogenes Laertius 9.4 (on Melissus; tr. R.D. Hicks):
Moreover he said that we ought not to make any statements about the gods, for it was impossible to have knowledge of them.
ἀλλὰ καὶ περὶ θεῶν ἔλεγε μὴ δεῖν ἀποφαίνεσθαι· μὴ γὰρ εἶναι γνῶσιν αὐτῶν.
Id. 9.51 (on Protagoras):
In another work he began thus: "As to the gods, I have no means of knowing either that they exist or that they do not exist. For many are the obstacles that impede knowledge, both the obscurity of the question and the shortness of human life."
περὶ μὲν θεῶν οὐκ ἔχω εἰδέναι οὔθ᾿ ὡς εἰσίν, οὔθ᾿ ὡς οὐκ εἰσίν·1 πολλὰ γὰρ τὰ κωλύοντα εἰδέναι, ἥ τ᾿ ἀδηλότης καὶ βραχὺς ὢν ὁ βίος τοῦ ἀνθρώπου.”
1 οὐθ᾿ ὁποῖοί τινες ἰδέαν Diels ex Euseb. P.E. xiv.3.7.
Robert Renehan, "Herodotean Cruces," Harvard Studies in Classical Philology
89 (1985) 25-35 (at 34-35):
It may thus be taken as established that, in an exposition of reasons,
introductory πολλά occasionally followed by only two reasons is
a correct Greek idiom, hitherto unremarked as it seems. The purpose
(and effect) of the idiom is of course to single out as most important
those two reasons which are explicitly mentioned....Protagoras wished to emphasize two
things, first, that there are many reasons for our lack of knowledge
about the gods, and, second, that the most important reasons were
two, the intrinsic uncertainty of the subject and the brevity of human existence. He achieves both purposes by employing a correct Greek
idiom, πολλά followed by the explicit mention of two reasons only.