This Article explores an underappreciated legacy of the Supreme Court’s (in)famous decision in Church of the Holy Trinity v. United States. Although Holy Trinity has been much discussed in the academic literature and in judicial opinions, the discussion thus far has focused almost exclusively on the first half of the Court’s opinion—which declares that the “spirit” of a statute should trump its “letter”—and relies on legislative history to help divine that spirit. Scholars and jurists have paid little, if any, attention to the opinion’s lengthy second half. In that second half, the Court tells a detailed narrative about the country’s historically Christian roots and explains that, other interpretive rules aside, the statute simply cannot be construed against the church because the United States “is a Christian nation.” This Article maps the methodology of the Holy Trinity Court’s Christian-nation argument and contends that that methodology constitutes an interpretive canon in its own right—one which perhaps aptly can be called the “unique national institution” canon. The Article goes on to demonstrate that this interpretative canon has reared its head in a number of statutory interpretation cases decided since Holy Trinity.