lpetrich

Among ancient Greek thinkers, I think the first was Xenophanes. He gets this honor because he pointed out that people create deities in their likeness -- the same physical appearance, even the same clothes.

He was followed by Protagoras, who wrote a book, On the Gods, in which he stated that he was not sure whether they existed or what they were really like if they did. However, he thought that the official gods ought to be worshipped, because in the absence of better knowledge, it's best to do what one had already been doing.

In the 300's BCE, a certain Euhemerus proposed the theory that the Gods were really human heroes exaggerated into divinity -- that Zeus was a king with an eye for the ladies, perhaps. This theory became very popular in the Greco-Roman world, though some competition like that of Theagenes, who believed that the Gods were allegories of natural forces -- Zeus is the sky, and he fertilizes the Earth with his rain. And Platonist-minded individuals had other allegory theories.

As to Plato himself, he proposed that his society's sacred books be banned from his Republic, on the ground that they contain such bad examples as heroes lamenting and gods laughing. They were to be replaced with an official religion/ideology that he called a "royal lie" -- one designed to demonstrate the legitimacy of that Republic's philosopher-rulers.

Some people were absolutely indifferent to religion, however, like the early historian Thucydides, who related the years-long war between Athens and Sparta in the 400's BCE.