Statistical data on levels of church attendance before the 1960s is fragmentary, but what there is suggests that the low levels of religious participation recorded later in the century were not so new. Clergy returns for Denmark, even excluding the metropolis of Copenhagen, where levels of church attendance were known to be exceptionally low, recorded that only about 8 percent of Danes attended Lutheran services on “an average Sunday” in the 1930s, falling to 6 percent in the 1940s, 5 percent in the 1950s, and 4 percent in the 1960s…In Sweden in 1927 it is estimated that only 5.6 percent attended a Lutheran church on a normal Sunday; by the 1950s it was under three percent.
A Norwegian estimate is of 3.8 percent in 1938, and Finland seems to have been below 3 percent by the 1950s. For purposes of perspective, in most other European countries the collapse of church attendance came in the 1960s.
That is all from the new and interesting Brian Stanley, Christianity in the Twentieth Century: A World History, published by Princeton University Press.