What constitutes a true church? This article reflects on the thinking of Martin Luther on the church, for he was so intense in the way he wrestled to find a biblical answer to his questions about the church. To understand what Luther understood a true church to be, it is necessary to understand what he meant by the terms "hidden"/"invisible," "visible," and "false." The main thesis of the article is that the church is not "invisible" in contrast to "visible." However, the church is "hidden," as a result of it being united to Christ.