One of the main differences between classical Greece and Medieval Europe was the ability for the Greeks to have logic and spirituality, logos and muthos, co-exist in such a way that the Gods were never threatened in Greek society by the empiricism of the Greek philosophers and scientists. Freeman writes:

One of the most sophisticated of the Greek intellectual achievements was the distinction between the areas of knowledge in which certainty was possible and those that were not subject to the rational.

A mathematical proof could be sustained by deductive logic and was unarguably true, while a myth was fluid and flexible, open to individual interpretation.

Myths had power but to the Greek mind the idea that a person could demand others believe a myth was true was absurd. In the Medieval world, power and politics conspired in such a manner that the Vatican and the successive rulers of the Holy Roman Empire were able to enforce through violence the requirement that myths were true and be treated as 'gospel'. Inquiry was treated as heresy and discouraged; the secrets of nature were wholly owned by myth, not reason. (more)
Lee Malatesta : On the one hand, one of the interesting things about most of the classical Greek philosophers is that they /rejected/ the myths of ancient Greece. Socrates was notable for not only rejecting them but saying that they had to be rewritten to leave all the bad behavior out, told as Noble Lies, and then made a mandatory part of education.

And on the other hand, we all know what happened to Socrates when brought up on charges of impiety and corrupting the youth.

I think a much more parsimonious explanation is that as civilization feel in many areas of western Europe, myth collapsed into reason. As an example, one of the early Christian emperors (I want to say Justinian but I could be wrong) banned the practice of burning witches under the rationale that witches had no real power. Fast forward a few hundred years and in parts of Europe they were burning witches again because they thought that the witches were doing them actual harm. Myth became reality.
I am currently reading The Closing of the Western Mind which charts how the reason and rationality of the Greek world was replaced with the dogma of faith in the Medieval world. The interfaces of rationality are fascinating; I recently read through A World Lit Only By Fire which looked at the Medieval mind and how it changed with the reformation and renaissance. I also finished Peter Watson's The Modern Mind which looks a the rationality of the twentieth century and how science came to dominate modern intellectualism. The rationality of humankind is an amazing thing.
John Barrdear : Without necessarily advocating his thesis, it might be interesting to compare with John Ralston Saul's "Voltaire's Bastards: The Dictatorship of Reason in the West". Wikipedia's surprisingly brief one-paragraph summary really is a one-paragraph summary:

These books deal with themes such as the dictatorship of reason unbalanced by other human qualities, how it can be used for any ends especially in a directionless state that rewards the pursuit of power for power's sake. He argues that this leads to deformations of thought such as ideology promoted as truth; the rational but anti-democratic structures of corporatism, by which he means the worship of small groups; and the use of language and expertise to mask a practical understanding of the harm this causes, and what else our society might do. He argues that the rise of individualism with no regard for the role of society has not created greater individual autonomy and self-determination, as was once hoped, but isolation and alienation. He calls for a pursuit of a more humanist ideal in which reason is balanced with other human mental capacities such as common sense, ethics, intuition, creativity, and memory, for the sake of the common good, and he discusses the importance of unfettered language and practical democracy.

cam : The Greeks recognised that rhetoric didnt need a moral rudder to be presuasive which is why Aristotle came up with the Philosopher King - a moral, learned individual able to rule in the public good.

It is probably also why liberalism's utilitarianism tries to temper it with morality and personal responsibility. That is where the social good comes in.
Cam Riley: South Sea Republic. Freedom, liberty, equity and an Australian Republic.