For humans to engage in a religion, they have to believe that it is more than the creation of their own ego. That it reveals Reality in a way that they could not otherwise know.
Rudolf Otto's description of the divine as mysterium tremendum et fascinans is apt. It is mysterious by nature and cannot in principle be comprehended by us. It is overwhelming in its power and scope. And it captures our attention, imagination and awe. Without these elements, it is not really sacred.
Thinking of the liberal rationalism which lies at the heart of post-Enlightenment faiths, such as Unitarian Universalism, I cannot see them as engaging the divine. They are just religious ideologies, having reduced their scope to what they can understand.
On the other hand, when history and culture change and a given religion can no longer make "good-enough" sense of things or provide a way of life that people can actually live --or are any longer willing to live-- its scope is likely to shrink.
I do not think it accidental that a European (and rapidly and increasing an American) civilization which has "emancipated" itself from the religion of its ancestors --Christianity, for the last many centuries-- now finds itself incapable of resisting the savages invading its borders. I think they are linked.
As I have said, the young reactionaries who have awakened to the imminent collapse of their people and blame Christianity's universalism for that fail to note that it was only after Christianity ceased to be the spiritus rector of their people that the sturdily evangelized cells of the Western body metastasized into the cancer of global liberal secular humanism, leaving us defenseless against the New Huns.
It is the emptied churches of the West that the Muslims are turning into mosques.