This article explores the connection between the ecclesiology and the beliefs on church-state relations of Baptists in the mid-twentieth-century United States. The author analyzes white Baptists’ reactions to the US Supreme Court rulings in Everson v. Board of Education (1947) and McCollum v. Board of Education (1948), both of which inaugurated the modern era of strict separationist Establishment Clause jurisprudence. The author also traces the development of Baptist beliefs on how the institutional church relates to individual salvation—beliefs that distinguished Baptists from both Catholics and most other Protestants—and statements from US Baptist leadership supporting church-state separation. The author demonstrates that Baptists’ beliefs on the internal, individualistic, and non-sacramental nature of salvation induced them to see any government-sponsored religious activity as likely corrupting of a person’s genuine choice of salvation. Furthermore, Baptists’ origins as a persecuted minority in Europe and the United States reinforced their idea that government-sponsored religion would lead to the suppression of true Christianity. For both reasons, then, state-sponsored religion was not God’s design. Beginning with Everson and McCollum and continuing with later cases through the 1960s, Baptist’s strict separationism became the binding interpretation of the First Amendment’s Establishment Clause through Justice Hugo Black, who authored both the Everson and McCollum majority opinions. Although no longer a Baptist when the rulings were issued, Black retained his Baptist influence on church-state issues and enshrined strict separationism into American case law for decades, leading to a Baptist triumph that many Baptists themselves would later regret and attempt to reverse.