We do know that the Left treats their worldview and politics as a religion. So it’s a good question as to whether “church” attendance is down if you count all the “Walks-for-life” jogs, walks, and marches, count all the ribbons worn for this or that cause, count all the protest gatherings, count all the environmental wacko stuff, count all the beliefs (often expressed inside of organized religion) that are de facto religious views.
Still, another issue is that it would be entirely expected and predictable that as our material comfort and safety increases, there would be less impetus to believe in god and take part in organized religion. This is true, right or left, up or down, black or white. Not all of religious observance is about “Please, Lord, saves us from hardship and the bad people.” A great deal of it is about gratitude. But I would expect as the hardship decreases, religious observance would decrease as well.
Add to that the stigmatization and ongoing attack on Christianity (the #1 religion by far in America) and cause gives way to effect. And more and more of the remnants of religion — the people who still do regularly go to church — have basically borrowed the existing structures and bureaucracy for the worship of their liberal beliefs.
And pushed sideways and perhaps to a bit of extremism by this are the authentic church-goers, many who no longer parse things through Christ, per se, but through right-versus-left (as superficially understood) and thus damage their own religion as well.