Having argued that philosophical epochs end when their main concepts become exhausted, Langer turns from Greek and Roman thought to the period of Christian dominance of Western thought.]
The human mind is always active. When philosophy lies fallow, other fields bring abundance of fruit. The end of Hellenism was the beginning of Christianity, a period of deep emotional life, military and political enterprise, rapid civilization of barbarous hordes, possession of new lands. Wild northern Europe was opened to the Mediterranean world. . . . The wonderful flights of imagination and feeling inspired by the rise and triumph of Christianity, the questions to which its profound revolutionary attitude gave rise, provided for nearly a thousand years of philosophical growth, beginning with the early Church Fathers and culminating in the great Scholastics. But, at last, its generative ideas—sin and salvation, nature and grace, unity, infinity, and kingdom—had done their work. Vast systems of thought had been formulated, and all relevant problems had been mooted. Then came the unanswerable puzzles, the paradoxes that always mark the limit of what a generative idea, an intellectual vision, will do. The exhausted Christian mind rested its case, and philosophy became a reiteration and ever-weakening justification of faith. 
[Next, according to Langer,] the Cartesian age of “natural and mental philosophy” succeeded to the realm. This new epoch had a mighty and revolutionary generative idea: the dichotomy of all reality into inner experience and outer world, subject and object, private reality and public truth. 
Langer, Suzanne K. Philosophy in a New Key: A Study in the Symbolism of Reason, Rite, and Art (1941). New York: Mentor/American Library, 1951.
Last modified 14 July 2019