I believe that is the the way it was here in the U.S. up to the 1960's. With regard to businesses bring closed on Sundays.I wouldn't call Germany secular. Many people pay not only income tax but an income-related Church tax but this can only go to either the Catholic or Protestant Church. There are many religious holidays in Germany, some Catholic, some Protestant. Bavaria has the most. But other religions are still being ignored.
The reason for everything being closed on a Sunday is probably because the unions are strictly against longer hours. There is also a general feeling that there should be at least one day a week when the family can be together - at least in theory.