Correction: Since the 1940s

"For the first time" is probably not true. This is since Gallup tracked this number. Post WWII church attendance really sky rocketed for reasons that are still not fully understood, but a lot of people think the emergence of the Evangelical movement bears a lot of the responsibility. It's much harder to track pre-WWII numbers, but according to the "Mapping America's Past" authors[1] church attendance was as low as 35% in the 19th century (sometimes).

[1] https://www.amazon.com/dp/0805049274/?coliid=I38DOW2LIAPT9J&...