And they found the details of their moral code in sacred texts and history, as well as custom

From Neil Postman's Building a Bridge to the 18th Century

What we may learn from these two great philosophes, Einstein and Mill, is what they learned from their predecessors - that it is necessary to live as if there is a transcendental authority. "One can have the clearest and most complete knowledge of what is," Einstein wrote, "and yet not be able to deduce from that what should be the goal of our human aspirations. Objective knowledge provides us with powerful instruments for the achievement of certain ends, but the ultimate goal itself and the longing to reach it must come from another source." The other source is religion. Neither Mill nor Einstein believed in the stories that give form and inspiration to traditional religious systems, what Mill called the "supernatural religions." But both understood that we require a story that provides a basis for moral conduct and has a transcendent character. They found it in "natural law," and in the capacities of "human nature." In their stories, human beings have innate feelings for the general good and the unity of mankind. Mill called his story The Religion of Humanity. Einstein spoke of Cosmic Religious feeling. And they found the details of their moral code in sacred texts and history, as well as custom; that is to say, in our obligations to those whom we have judged to have acted in accord with the principles of human solidarity. Mill wrote:

. . . the thought that our dead parents or friends would have approved our conduct is scarcely less powerful motive than the knowledge that our living ones do approve it; and the idea that Socrates, or Howard, or Washington, or Antonius, or Christ would have sympathized with us, or that we are attempting to do our part in the spirit in which they did theirs, has operated on the very best minds as a strong incentive to act up to their highest feelings and convictions.

That there is a tendency as part of our nature toward our being "moral" - detesting wanton killing, honoring parents, caring for children, speaking truthfully, loving mercy, overcoming egotism, and all the other exhortations we find shared by sacred texts - is a legacy of the Enlightenment. And that this tendency cannot be proven in a scientific manner but must be taken on faith is also a feature of that legacy, provided that one does not claim absolute certainty for one's belief. For it is clear that most Enlightenment philosophes understood that absolute certainty is an evil that chokes reason and perverts faith; it is, in fact, the opposite of the religious spirit. They did not, therefore, find it necessary to have it "proved" that their narrative is certain, or superior to all others, or logically unassailable. Their narrative had only to be sufficient to guide them to a path of righteousness as defined by reason and historical agreement. The modern Christian apologist C.S. Lewis refers to "historical agreement" as the Tao, the summary of commands and prohibitions found in all collections of moral discourse from ancient Egypt to Babylonia to the Chinese analects to Homer's Iliad to the Old and New Testaments. The eighteenth century could not have used the term "Tao," but this is what eighteenth-century thinkers meant. This is what was "self-evident." And this is what provided courage and optimism.