The phrase "large congregation" refers to a big group of people gathering together for a particular purpose, usually for religious worship or a social event. Full definition
There are large congregations with social views similar to the author's. (religion.blogs.cnn.com)
As a result, our written materials require a college education to understand, denominational leaders are drawn from large congregations only, seminaries train ministers to work mainly in large churches, and even presbyteries devalue those who work and worship in small - membership churches by rarely making use of their leadership. (religion-online.org)