There is an ongoing debate about whether the U.S. Constitution includes – or should be interpreted to include – a principle of “church autonomy.” Catholic doctrine and political theology, by contrast, clearly articulated a principle of ”libertas ecclesiae,” liberty of the church, when during the nineteenth and early twentieth centuries the Church differentiated herself from the state. This article explores the meaning and origin of the doctrine of the libertas ecclesiae and the proper relationship among churches, civil society, and government. In doing so, it highlights the points at which church and state should cooperate and the points at which mutual assistance would be ultra vires.