I think Stephen is wrong here. Ancient minds are fully penetrable; and I maintain this because from what I can see the mind of man has changed very little over the past five or more thousand years of human history.
The external trappings have changed, to be sure; mythologies have permutated, Gods have come and gone, and—above all—technologies have flourished, allowing man to fully transform his relationship with the natural world.
But these are external matters. Man's behavior has, I would say, improved not a whit over the entire length of his sojourn as a "civilized" being; what has improved, above all, is his ability to manipulate materials and kill others. We mistake progress in technology for superiority over earlier cultures and minds; yet real progress, if there ever were any, would consist of improvement in man's spiritual and ethical behavior, not his technologies. The consistent record of escalating environmental destruction—which is emphatically not a sign of intelligence, but rather the opposite— and the longstanding track record of rape, torture, murder and their institutionalized versions, collectively called war, demonstrate a remarkable and entirely distressing consistency that argues powerfully for a psychological and spiritual consistency that runs as a powerful thread throughout man's history, regardless of the colors of its outer wrappings.
Given the convergent nature of man's biology and his psychology, we can rely, more or less, on the idea that what ancient men were thinking, after we tear off the outer layers of the onion, is much like what man thinks today. We can, therefore, intuit thought forms and patterns which underlie and inwardly form all of the outer activities which may seem, again on the surface of things, to be very different and (if ancient and undocumented, as with for example the Indus River Valley Civilizations) inscrutable.
We assume ancient cultures to be inscrutable, whereas I would argue they are very scrutable indeed; ancient people, despite their lack of apparent technology (much of which was actually very sophisticated indeed, but based on natural and perishable materials) thought and acted very much the same as we do today, and we can draw many conclusions about them simply by mirroring our own ideas in the remains of their art, mythology, and technologies.
So we can, I would say, enter the minds of ancient peoples; and they are our own minds, which we fail, now as then, to sufficiently scrutinize. Mankind lives in a collective state of denial in which today's—and yesterday's—actions are allowed to go unexamined on the deeper levels, which is where the examination ought to begin if we want to understand anything properly. We are a collective race of beings; Carl Jung's proposition—a collective unconscious—is at best misunderstood and more often (worse) forgotten. People like to give emphasis to the word collective when thinking about Jung's idea; and this is because the idea of a great collective that binds all Beings together into a single, secret whole is so appealing and touchy-feely, in a new-age kind of way. "We are all one," we'd like to say; as if this magical talisman could somehow trump ten thousand years of warfare and create the kumbayah moment where we finally, for once, actually like (or even love) one another.
Yet it is mankind's unconscious that forms the center of gravity around Jung's idea; and it is this very unconscious property of man's awareness and Being that forms him, more than anything else. It is, in a nutshell, Gurdjieff's sleep, that property whereby both individuals and collectives stumble drunk from one lamppost to another, congratulating each other on their progress.
In any event, when we romanticize ancient peoples, their cultures, and their beliefs, we subtly remove them from ourselves. The proposition that their mindsets and mental processes are alien to our own is a subconscious way of asserting our superiority; an imperialism not of location, if you will, but time, whereby today's humanity supposedly represents a better version of humanity than what was had some few thousand years ago. Viewing historical humanity through this distorted lens also tends to make ancient people more exciting; it's the otherness of people that appeals to us, the perception that they were—or are, in the present case— different than we are. This delusional belief in some fundamental difference, which is imposed almost exclusively by externally imparted cultural values, belies the fact that human beings are, both biologically and psychologically (the two cannot actually be separated, since our psychology is a part of our biology) very nearly identical across the spectrum. Cultural filters such as caste systems or religions create differences which are for the most part arbitrary and superficial, despite their apparently compelling features. Mankind keeps coming back, over and over, to a very similar set of values that are somehow essential to our psychology: that there is an inherent good, and that we can strive for it, for example.
One might hope that, in a convergent environment, there is both a steady urge and a reliable evolution towards the good; these features have been a persistent enough thing throughout cultures across time to be considered as such.
Nonetheless, we ought, I think, to hope and pray for more goodness than we have; else what is prayer for in the first place?