Hello Phaedrus! WRT the ancient philosophers, I think this article's distinction of esoteric and the exoteric knowledge is on-point. But that is not to say that esoteric knowledge is premised on the idea that there is no God. Quite the contrary.
By all indications Plato, for instance, was in favor of the Olympian cult for the masses, for the cult was invariably associated with the common self-understanding of the polis. Thus he deemed cultic faith as essential to the maintenance of social order. Indeed, in his own day Plato was concerned that the social order was falling apart, because the traditional cult of the polis was increasingly being displaced by "strange gods" brought to Athens by virtue of its commercial and military contacts with cultures outside of Greece.
Further, I gather he thought most people have neither the time, interest, nor ability to penetrate the deeper strata of divine truth. That was the special province of the philosopher -- i.e., of the very few. Plato was aware that what he knew was perfectly inaccessible to "the average person."
To postulate that Plato, or Aristotle after him, were "closet atheists" trying to conceal from the masses the fact that there was no god in order to preserve the social order simply cannot, IMHO, be corroborated on the basis of their extant works.
Fascinating article, general_re. Thank you for posting it.
BB! It's difficult to use the term "atheist" in the context of Greeks like Plato and Aristotle, as they were generally unfamiliar with the concept of one God, which was then believed only by the Jews. Indeed, it was only in Aristotle's time, because of Alexander, that the Jews were liberated from Persian domination and began to participate fully in the world of Western civilization. Monotheism took hold in the West from that time forward. To the clasical Greeks, whose religion was a hodge-podge of myths about numerous gods, it may well be that an educated philosopher regarded the whole mess as silly. And there was the example of the trial and death of Socrates to chill any philosopher from being too outspoken, so they may well have been cautious in their public utterances.