The secular creep: Using the General Social Survey, I averaged the percent of Americans who never attend church:
Percent never going to church
So the increase in church avoidance is up 46% for whites and 57% for blacks over (roughly) 3 decades. One in five whites never attends religious services. America is still one of the most religious countries in the world (with indicators similar to Ireland, Poland, and India) but the drift is in a European direction. I suspect that the trend has less to do with growing disbelief than with diminishing appeal and sense of obligation, and later I'll see what the data say.