Embedded in U.S. legal frameworks governing the free exercise of religion is a criterion that has received surprisingly little theoretical attention: sincerity. Only those professions of belief that are sincere warrant legal accommodation. Nearly all of the existing literature on sincerity focuses on judicial “tests,” or evidentiary frameworks, for judging sincerity. This paper, in contrast, interrogates the notion of sincerity itself, developing a conception of what properly constitutes a sincere profession of belief. That conception includes three elements: genuineness, nonopportunism, and intelligibility. I then consider a fourth potential component of sincerity, vigilance, which concerns a believer’s consistency in living in accordance with their belief. A number of authors have recently proposed judicial tests requiring some sufficient degree of vigilance, but I argue that a vigilance criterion is incompatible with the fundamental values and objectives that underwrite the commitment to religious liberty in liberal political orders.