in Wil Durant's The Story of Civilization, he makes the case at some length that the educated, upper class, intelligentsia of Greece and Classical Rome were largely agnostic, or at least familiar and not especially surprised when they heard that point of view.

To make an enormous generalization from a distance of 18-25 centuries, the more educated elements saw the pantheon of Greek/Roman gods with their petty soap-opera lives as unlikely to be concretely factually correct. The stories might create a body of useful tools to teach people virtue or explain why we just lost the most recent big battle, or there's a drought, but they're not really real, as in real.

He does not seem religious by his writing, but he also made the case that this created a power vacuum that helped Christianity take hold.

I wish I could give you, or find, data. Durant just gives anecdotes because that's all one really has from this distance. To his credit, it's a whole lot of anecdotes. His books on the shelf look like the Encyclopedia Britannica.