Government neutrality toward religion is based on familiar considerations: the importance of avoiding religious conflict, alienation of religious minorities, and the danger that religious considerations will introduce a dangerous irrational dogmatism into politics and make democratic compromise more difficult. This Article explores one consideration, prominent at the time of the framing, that is often overlooked: the idea that religion can be corrupted by state involvement with it. This idea is friendly to religion but, precisely for that reason, is determined to keep the state away from religion. If the religion-protective argument for disestablishment is to be useful today, it cannot be adopted in the form in which it was understood in the seventeenth and eighteenth centuries, because in that form it is loaded with assumptions rooted in a particular variety of Protestant Christianity. Nonetheless, suitably revised, it provides a powerful reason for government, as a general matter, to keep its hands off religious doctrine. It offers the best explanation for many otherwise mysterious rules of Establishment Clause law.