Common knowledge has it that the decline of Mainline denominations began in the 1960s. There is little dispute that membership counts had been moving upward until that decade and then began to decline.
However, Roger Finke and Rodney Stark in The Churching of American, 1776-2005: Winners and Losers in Our Religious Economy suggest that Mainline decline far predates the 1960s. It is not enough that a denomination grow. It must grow at a pace equal to or faster than the host population to avoid decline. Looking at data from the past 230 years, the authors demonstrate that the Methodist decline actually began around 1850 when circuit riders were exchanged for more settled educated clergy and efforts were made to be accepted by those with status in society. Here is how the authors chart the history of the Methodist and Baptist families of denominations.
You can see the Methodists ceased keeping up with the population 150 years ago. This is also true for most, if not all, of the other Mainline denominations. The Baptists have been expanding faster than population until very recently.
The next two graphs show you the trends for some Mainline denominations and Evangelical denominations over the last sixty years.
The size of the Southern Baptist group makes it hard to see what happened with smaller denominations. The Nazarenes grew by 63% between 1940-2000. The other groups grew by 500% of more.
It does indeed appear that Mainline decline is far earlier than we usually think if you subscribe to Finke and Stark’s viewpoint.