Here are two fragments from the lost Euripidean Bellerophon in which the eponymous hero denies that the gods exist. He does not seem to say that there are no gods at all, but his complaints are like those of Xenophanes who complains about the misbehavior of Homer’s gods.

Instead, Bellerophon’s complaints are based on the fact that since the world seems unjust and the gods are supposed to ensure justice, therefore they must not exist (either totally or in the form man makes them).

Things turn out badly for Bellerophon, as one might imagine.

Euripides, fr.286.1-7 (Bellerophon)

“Is there anyone who thinks there are gods in heaven?
There are not. There are not, for any man who wishes
Not to be a fool and trust some ancient story.
Look at it yourselves, don’t make up your mind
Because of my words. I think that tyranny
Kills so many men and steals their possessions
And that men break their oaths by sacking cities.
But the men who do such things are more fortunate
Than those who live each day piously, at peace.
I know that small cities honor the gods,
Cities that obey stronger more impious men
Because they are overpowered by the strength of their arms.”

φησίν τις εἶναι δῆτ’ ἐν οὐρανῷ θεούς;
οὐκ εἰσίν, οὐκ εἴσ’, εἴ τις ἀνθρώπων θέλει
μὴ τῷ παλαιῷ μῶρος ὢν χρῆσθαι λόγῳ.
σκέψασθε δ’ αὐτοί, μὴ ἐπὶ τοῖς ἐμοῖς λόγοις
γνώμην ἔχοντες. φήμ’ ἐγὼ τυραννίδα
κτείνειν τε πλείστους κτημάτων τ’ ἀποστερεῖν
ὅρκους τε παραβαίνοντας ἐκπορθεῖν πόλεις·
καὶ ταῦτα δρῶντες μᾶλλόν εἰσ’ εὐδαίμονες
τῶν εὐσεβούντων ἡσυχῇ καθ’ ἡμέραν
πόλεις τε μικρὰς οἶδα τιμώσας θεούς,
αἳ μειζόνων κλύουσι δυσσεβεστέρων
λόγχης ἀριθμῷ πλείονος κρατούμεναι.

Euripides, fr. 292.6 (Bellerophon)

“If the gods do a shameful thing, they are not gods.”

εἰ θεοί τι δρῶσιν αἰσχρόν, οὐκ εἰσὶν θεοί.