There is an old story told about the evolution of human thought that goes something like this: first there were many gods, then there was one, and now there are none. The story has a certain conceptual elegance and persuasive proportionality. It also helps that it conforms easily to a general account of what is called Western Civilization by those with the advantage of hindsight. The parts of the story that took place in Ancient Egypt and Mesopotamia hardly can hardly be called Western, and periodically so-called 'Western' civility breaks down into brutal expressions of inhumanity. But these inconveniences are neither here nor there.
Ancient Egypt Greece, Babylon, and Rome knew many gods. Christianity took over from its parent Judaism, which had existed between Egypt and Mesopotamia, the idea of a single transcendent God. Eventually Christianity burst the boundaries of the Roman Empire, establishing in Europe a rancorous patchwork of states joined loosely together by the idea of Christendom presided over by a pope. When, in the 15th, 16th, and 17th centuries, the loose unity of Christendom was torn asunder; every polity did as it saw fit.
Following the 17th century European Wars of Religion, if 19th century humanists are to be believed, the many ideas of one God lay scattered about a triumphant humanity, which held the field. We might quibble about the century in which humanity uncovered its sensus humanitatis from beneath the oppressive weight of the sensus divinitatis. We may even wonder, in fact, if it was recovered at all. Regardless, influential later thinkers towards the end of the 19th century did believe something like this had come to pass. Jakob Burckhardt detected the origins of this nascent humanism as early as the Italian Renaissance. One of his more troubled contemporaries by the name of Nietzsche would lament, "Ecce Homo" ("Behold, the Man"), a phrase lifted from St. Jerome's Latin Vulgate Bible, specifically his translation of the Gospel of John 19:5.
A decade into the 21st century, the historical vista is much wider than it was for our 19th century counterparts. Many gods giving way to one God, who yielded the field to a triumphant humanity, is a story that fits a little too snugly into a Anglo-Franco-German (North American and Northern European) perspective on the course of human history. There are also the great civilizations of China and India, for which some account must be given. So we debate the finer points of even this very general picture.
There remains a need to account, in a very general manner, the way in which contemporary Western civilization, which has demonstrated in mastery over the natural world via scientific methods of study and technological innovations, stands apart from the medieval Christendom and Islam, as well as ancient China and India.
Let's say one is inclined to think that cultural comparisons of religious convictions about the existence of a transcendent Something are too tenuous to infer anything about the general direction human affairs seem to be moving. Then we can still look at the commonly shared system of government across the ancient and medieval worlds: monarchy. The rule of the One over the many, a cosmological scheme, was recapitulated in the social order. Kings ruled on behalf of Divinity, or kings ruled because the distinction between ruler and subject was woven into the natural order of things. It doesn't matter which shoe fits, because both shoes take us to the same location.
Whatever else the secular outlooks of the modern Western world might share with, say, Buddhism or Confucianism, that they don't share with Christianity or Islam, secular outlooks distinguish themselves from all such classical forms of religion by being corrosive upon the claims of monarchy. Authorities today may prance around and behave like monarchs of old--they may even behave worse--but they cannot justify their rule in the same way that their fore-bearers did. The Middle Eastern kingdoms of Saudi Arabia and Jordan only appear as exceptions to this rule; their right to rule is justified by oil wealth, without which they could hardly command the allegiance of subjects that ancient and medieval forms of government were based upon.
So what exactly sets the ancient and modern worlds apart? The explanatory successes of natural science can be offered as an explanation. My own take on the situation, however, is that the successes of natural science are a by-product of an intellectual revolution, through which the world has quite literally been turned inside out. We no longer look up and wonder who or what is looking down on us. As a consequence, we no longer wonder about the standard against which our actions will be measured.
Now we look down on ourselves from an artificial distance, cut ourselves into tinier pieces, which we plug into flowcharts, diagrams, and pie graphs. And we chide ourselves for thinking someone or something might have been doing the same.