What I thought: 700 referred to the religion section of the Dewey decimal system. (Religion is actually the 200 section.)
What it is actually named after: "To keep the station on the air, WYAH produced a special telethon edition of the show. For the telethon, Robertson set a goal of 700 members each contributing $10.00 per month, which was enough to support the station. Robertson referred to these members as the '700 Club' and the name stuck."