Unknowledgeable, by our standards, doesn't mean that the ancient Greeks, or any similar culture, were less intelligent than us. The Allegory really doesn't build off anything, so it's entirely up to the individual's intelligence, not the state of his society. I'm not even sure ours is all that much superior, anyway. You mentioned a religious belief that contradicts what we now know as scientific fact, but today we still have people who believe very similar things, even when the knowledge is available to them. I'd argue that's far worse.

Yes, but the amount of unreasonable beliefs back then was significantly greater than it is now.  We still have people who know little and believe unsupported and even refuted ideas, but it is much less common today than it was back then.  My point was simply, that to have the mental facility to sift through all that foolishness surrounding him and make a conclusion like this is impressive, and I would say slightly more impressive to have done it then than to have done it today.

Also, yes, I agree, those few people who still believe that silliness despite the available knowledge are even worse.  They just exhibit even more egregious ignorance.