Religious arbitration agreements, which compel parties to resolve their legal disputes through religious tribunals, are increasingly common. This Article argues that judicial enforcement of religious arbitration agreements may violate the Establishment Clause’s nondelegation doctrine. First articulated in Larkin v. Grendel’s Den, the nondelegation doctrine prevents the government from making a delegation of its power to religious institutions that results in a “fusion” of governmental and religious functions. The concern is not that the government would entangle itself in religious affairs, but that religious institutions would acquire unilateral control over a properly governmental function. In particular, religious arbitration agreements allow religious tribunals to acquire unilateral control over the resolution of civil law disputes. The Article argues further that the religious question doctrine supports the conclusion that courts should not enforce religious arbitration agreements when the underlying civil law dispute turns on a legal, rather than a religious, question. The Article concludes by arguing that the principle of government neutrality toward religion also supports the nonenforcement of religious arbitration agreements because of the need to maintain a stable separation between church and state.