Peter Forrest argues that Essential Catholicism is the one and only religion to which reasonable humanists could commit. Drawing on Immanuel Kant's Religion within the Bounds of Bare Reason and Ludwig Feuerbach's The Essence of Christianity, he provides an in-depth philosophical investigation into the synthesis of Catholicism and the Enlightenment.

In this nuanced study humanity and divinity are two species of the same genus. Chapters defend the apologetic project from common objections and cover the argument from evil to Catholicism, the case for the Catholic synthesis, the argument from tradition, and finally the negative case for Essential Catholicism. Forrest elucidates how the various Catholic churches have Essential Catholicism in common, discussing the Roman Catholic, the Eastern Orthodox, the Oriental Orthodox, and the Old Catholics, as well as High Church Anglicanism with Methodism as a borderline case. He argues that we can learn from other Christian denominations and from other religions, but Catholicism is the overarching and one true religion.

An engaging and original account, The Essence of Catholicism departs from Vatican theory and provides a sharp philosophical reflection on a distinctive religious position within the Catholic tradition of thought.