Benjamin Constant's (1767-1830) De la Religion, consideree dans sa source, ses formes et ses developpements (On Religion, Considered in its Origin, its Forms, and its Developments) (five volumes, 1824-31) is one site of the construction of the powerful modern idea of religion as private. This paper analyzes Constant's liberal and idealist conceptualization of religion, not as a program or method to be emulated today, but for the illuminating perspectives the epistemological and historical investigation of it can bring to the status of religion in liberal democratic societies today.