Religions are in decline, not necessarily theistic belief.
Confusing the two is common, in Western societies.
(It's common to underestimate the prevalence of atheists among the ostensibly theistic religious, in the first place - 4-12% of Catholic priests, for example).
The US is returning to its roots. In 1776, only 17% of the population was active in any religion
That would have little to do with theistic belief.
It's quite clear. Theism will continue to decline as individualism and individual rights increase, and as conservative social traditions falter.
Unless, say, Islam takes over. Consider the possibilities under the increasing chaos of climate change, if high-tech societies fail to protect their institutions and accumulated wealth of knowledge.
The hearings on providing a US Supreme Court majority (joining a Congressional majority and an Executive endorsement) for this agenda:
https://www.motherjones.com/politic...schools-vouchers-charter-education-secretary/
begin shortly.
Organized religion has not adjusted to technological and scientific change, or incorporated the bulk of recently acquired knowledge. It no longer fulfills its function, does its job, in maintaining good relations between complex physical reality and human society - even in the basics: even modern Tragedies of the Commons go unrecognized, go unopposed and unmoderated let alone prevented if possible, by modern religions.
Antibiotic resistance, for example, has not been addressed by any modern religion - it is a moral and ethical failure, involving self-sacrifice according to wisdom, of a kind central to organized religion in its traditional role.