Although the Essay Concerning Human Understanding is considered Locke'smagnum opus, its relation to his political philosophy has been a perennial puzzle for scholars. Scholars have typically focused on the question of Locke's natural law doctrine in the Essay and the Two Treatises. This article takes a different approach to uncovering the political significance of the Essay by relating the theological importance of its epistemology to Locke's doctrine of religious toleration as found in the Letter Concerning Toleration. Crucial arguments supporting Locke's case for religious toleration are to be found, not in the Letter, but in the Essay, which seeks to elevate a limited but relatively uncontroversial natural theology above the disputed uncertainties inherent to revealed theology. Though not on the surface a political work, the Essay is fundamental to Locke's political theology of toleration.