The United States is more secular now than it has been at any point the early 1800s before the Second Great Awakening (ca. 1790-1860 CE, flourishing from about 1820-1855 CE) that gave birth to the uniquely American variety of Evangelical Christianity.
The United States now has tens of millions of secular people, who more or less independently came to the same decision to leave Christianity (for the vast majority of secular Americans are former Christians), at about the same time, for similar reasons, on a grass roots basis.
But, they are collectively faced with an omnipresent question, "now what"? How shall I live my life from day to day? What traditions shall I honor? What do I believe? What is moral and what is not? Into what worldview to I frame my experiences?
We have models abroad for how to live in a more secular society.
Europe experienced a similar surge in secularism a generation or two before the United States did, and is now to a great extent a post-Christian society for whom the state religions of their ancestors are now shared cultural legacies, rather than faiths lived on a day to day basis. Churches are empty, or filled with immigrants and the elderly who haven't experienced a similar transition.
Japan, while not precisely a secular society, has also never been dominated by Christianity or Islam. Individual Japanese individuals can transition from a state in which superstition was taken more seriously, to one in which superstition is taken less seriously, without as much of an abrupt change of worldview, like a child gradually outgrowing childhood superstitions, one by one. Japan provides another model of how to have a society that does not live in the shadow of Christianity.
Communist countries that once made up much of the world also consciously and actively collectively disavowed religion, and now that Communism had become obsolete in Europe and much of the rest of the world, and become untethered from its intellectual foundations in places like China, religion is creeping back. But, a multitude of former Communists have not returned to religion after abandoning the religion of Communism and now face a religious vacuum of their own in which they too must craft of new set of folkways.
The process of creating new folkways is democratic in the small "d" sense of the word. No choices about how to live are neutral and so everyone must participate in formulating their own, whether or not they want to do so, both individually and in relation to larger, often grass roots, social movements.
This transitional period won't last forever. Folkways, traditions and norms that are congealing into a new culture right now will solidify over time and may have become rigid for an indefinite future of many centuries within a few generations from now.
It will be natural to answer many questions, particularly as they pertain to metaphysics, with science: the evolution of all life from a common ancestor billions of years ago; an understanding of biodiversity and neurodiversity informed by genetics, the Big Bang and its associated cosmology, a universe where everything is governed by laws of nature exemplified by the Standard Model of particle physics and general relativity, or their successors, a blurring of the mind-body duality, an understanding of how much of what happens in the world is random or at least amoral.
Other questions aren't as amenable to consensus scientific answers. Science can tell us a lot about gender and sexuality, but it can't tell us, by itself, what kind of personal and intimate relations to develop, even if it can inform our understanding of the choices we make. It can't tell us how to govern ourselves or raise our children, even though its methods can inform those choices as well. It can't tell us who to trust, or who to love, who to chose as allies, or what battles and wars we should wage. It can't tell us when to forgive and when to bear a grudge. Even if science and the scientific method can inform many of these aspects of our lives, it hasn't reached that point yet, and the questions we face in life must be answered, now, immediately, with the knowledge that we have and not the knowledge that we hope some day as a species to acquire.
Even if almost everyone becomes effectively non-religious, there is no reason to think that we will not have, as the Ancient Greeks and Romans did, competing philosophies of life that each claim large numbers of adherents and guide the lives of those who follow them. It isn't obvious at the moment what the competing philosophies will be, but it seems to me a near certainty that they will emerge over some point that is unknowable in the present in some respect that does not decisively disadvantage one view over another. I wouldn't even be surprised if the new competing philosophical schools track to a significant extent historical patterns of regional culture, religion and political identity.
Perhaps, for example, secular people will fall into camps of optimists and pessimists, with pessimism gaining more steam in places like the former Communist states and the American South, and optimism finding more purchase in Western Europe, the Northeast Corridor and the Pacific Coast of the United States. Science can certainly not tell us if optimism is preferable to pessimism, or visa versa, and each outlook offers certain advantages in life to its adherents. Ideas like the Gaia hypothesis might be embraced by optimists, for example, while pessimists may breath new life into the Medea hypothesis, since it better fits with their philosophy.
Or, perhaps instead, we will be split between communitarians and libertarians, or between humanists and nihilists, or between people committed to "traditional humanity" and transhumanists.
I don't believe that there is nothing new under the sun. Cultural innovators among those of us cursed to live in this exciting time may, faced with the necessity of innovation, may come up with new kinds of relationships, new kinds of foods, new kinds of celebrations, and new kind of rituals and beliefs. The more untethered we become from our cultural legacies, the more room there is (and need there is) for cultural innovation.
Rivals of the Hebrew and Navajo language have required mass invention of new words to describe concepts absent from the historic versions of these languages, and a changing world will likewise require cultural innovation to deal with unprecedented realities and circumstances.
The difficult but worthwhile part of the venture it to image and guess what could emerge. The results will seem obvious and elementary and inevitable in hindsight, but are anything but if we try to imagine the outcomes prospectively with any degree of confidence.