I suspect one factor in the declining membership is shared with other mid-century social groups that have also dwindled (e.g., men's lodges): it's easier to find community elsewhere. For my parents -- born in 1940 -- church and men's service clubs/lodges were the primary areas of social interaction outside work. Dad ran his own veterinary practice, so his social interaction there was limited (I mean, everybody worked…
Men's lodges, fraternal organizations, and so on around here tend to have a religious underpinning of specific faiths, or seem to be charities of some kind with a connection to religion. As such, I think declining church membership would drag them down as well. I wonder if men's clubs exist which are neither, just completely secular. I haven't been able to find any when searching, aside from The Club, which technical…
Freemasonry may or may not be "religious" depending on your definitions, but has traditionally been a target of some faiths due to perceived conflict between the club and the church.