Three recent surveys have put the number of people in the US with no religion as 21%, 25%, and 32%. The usual explanation for the broad spread is that people are still sorting out how they feel about religion, so their answer is quite sensitive to how the question is asked. The Cooperative Election Study offered respondents three separate possibilities: atheist, agnostic, and "nothing in particular." It fascinates me that "nothing in particular" was one of the most common responses at 21%, about the same as Catholic or Evangelical. (Atheist got 6%, Agnostic 5%) This suggests to me that the majority of "nones", as pollsters call them, are people who just never think about religion at all.
You might think that the decline is focused in young people, especially young liberals, but that is not so; faith is declining is every ethnic, age, economic and political category. Pew's data looks like this:
The decline over the last decade in the share of Black (-11 percentage points) and Hispanic adults (-10 points) who are Christians is very similar to the decline among white adults (-12 points). The number of college graduates leaving the faith (-13 points) is similar to those without degrees (-11 points). The decline in organized religion is indeed much bigger among Democrats (-17 points) than Republicans (-7 points) and among Millennials (-16 points) compared to Baby Boomers (-6 points), but the trend is very broad.
The CES survey (remember they found the largest percentage of "nones") shows even less variation across these groups, and in particular almost identical declines in every age group.
None of these surveys finds any evidence that the decline is slowing; CES finds it accelerating, with numbers that make "nones" the majority in a decade. Gallup just reported that less than half of Americans belong to a religious congregation of any sort, for the first time ever, so maybe that makes sense. There is plenty of data showing that faith fades rapidly among those who have no religious affiliation.
As to where this will take us, I think we can already see. The right wing will become less traditionalist and more nationalist, or Trumpist. For a decade at least Republicans will continue to support "religious freedom" to keep Evangelical votes, but what happens after that is unclear to me; a split between tough-guy nationalists and people who believe in faith and old-fashioned ("classical") education seems plausible.
Meanwhile religion's substitutes will continue to appeal very broadly but especially on the left: anti-racist wokeness is one powerful version, and various groups are trying to make longing for a Revolution another.
The one thing I do not think will happen is that a decline in religiosity will lead to a major increase in rationality. Or happiness.