A novel account of the relationship between sincerity, religious freedom, and the secular in the United States. "Sincerely held religious belief" is now a common phrase in discussions of American religious freedom, from opinions handed down by the US Supreme Court to local controversies. The "sincerity test" of religious belief has become a cornerstone of US jurisprudence, framing what counts as legitimate grounds for First Amendment claims in the eyes of the law. In Sincerely Held, Charles McCrary provides an original account of how sincerely held religious belief became the primary standard for determining what legally counts as authentic religion. McCrary skillfully traces the interlocking histories of American sincerity, religion, and secularism starting in the mid-nineteenth century. He analyzes a diverse archive, including Herman Melville's novel The Confidence-Man, vice-suppressing police, Spiritualist women accused of being fortune-tellers, eclectic conscientious objectors, secularization theorists, Black revolutionaries, and anti-LGBTQ litigants. Across this history, McCrary reveals how sincerity and sincerely held religious belief developed as technologies of secular governance, determining what does and doesn't entitle a person to receive protections from the state. This fresh analysis of secularism in the United States invites further reflection on the role of sincerity in public life and religious studies scholarship, asking why sincerity has come to matter so much in a supposedly "post-truth" era.