What did Edward Gibbon mean by the triumph of barbarism and religion?

In his monumental work, The Decline and Fall of the Roman Empire, Edward Gibbon used the phrase "the triumph of barbarism and religion" to epigrammatically summarize his view on the primary causes behind the decay of the Greco-Roman world.1

Writing from the perspective of an 18th-century "philosopher," Gibbon interpreted the fall of the Roman Empire as a period of profound retrogression. He viewed the Roman Empire as having been built upon ideals of political and intellectual freedom found in classical literature. In his analysis, the rise of Christianity—which he treated as a phenomenon of human experience rather than divine revelation—and the subsequent influx of "barbarian" forces represented a departure from these classical ideals.12

For Gibbon, the material collapse of the empire served as both a symbol and a consequence of this moral and intellectual decadence. By characterizing this transition as a "triumph," he highlighted his belief that these forces had effectively dismantled the stability and rational culture of the ancient world.21