Demographic information for congregations is readily available, sometimes free and sometimes for a price. The Association of Religion Data Archives (ARDA) is the best free website of religious demographic information.
Online since 1998, this site has a home at Pennsylvania State University and is directed by Dr. Roger Finke. This website is useful for congregational leaders seeking data to conduct an external assessment of their context. You can find United States congregation membership data at the county level. The site posts results from a variety of religion surveys.
An especially useful tool is the community profile builder. This guide helps congregational leaders access free online information about their communities (income of households, education level of people within a particular zip code and so forth). There is a “QuickList” feature that allows you to find up-to-date data about particular denominations. This is a go-to site when you have a question (or are asked a question) about religion in America (“how many churches are there in Illinois?”). The feature called “Ahead of the Trend,” prepared by David Brings, a former writer for the Associated Press, features blogs about religious issues that are gaining in importance. “The Learning Center” hosts a variety of learning modules.
Before you pay for demographic information, explore this free and comprehensive service http://thearda.com/.