Despite scholars’ proclivity to identify natural religion with Enlightenment spirituality—the belief system of Shaftesbury, Hutcheson, and Voltaire—orthodox Protestants in the early modern period similarly understood that legitimate religious beliefs issued from non-textual sources and non-Christian peoples. Protestants of an evangelical, moderate, or rationalist persuasion all knew that nature and reason, or observations drawn from human history and from the created order, induced rational souls to see the reality of certain religious principles: the being and perfections of God, divine providence, the immortality of the soul, and a future state of reward and punishment. This dissertation explores how eighteenth-century American Protestants understood and applied this doctrine, and it argues that these Christian believers were not only comfortable with spiritual truths sourced in reason but often appealed to natural religious principles as a form of public morality and civil religion.