As to Sunday school, I've read somewhere, a long time ago, that Sunday school was originally intended (and so named) for kids or adults who couldn't attend ordinary school during the work week. It was offered as a service to those who would not otherwise get any schooling.

Because it was offered by churches or other religious-based institutions, the "commercial", so to speak, was the religious training offered along with the reading and writing and 'rithmetic. When the "auxiliary school" function became obsolete or at least less important, the "commercial" turned into an "infomercial" of just the religious message.

Dave Oesterreich