Tormented by growing economics hardship and insecurity, the urban poor of the third and fourth century empire turned more and more from the boisterous and unlikely gods of the Greco-Roman Olympic cult - Jupiter, Juno, Apollo, Minerva and the rest - to compelling new religions that offered release from individual guilt and the promise of personal salvation and eternal life. These new 'mystery religions' originated in the older, Eastern cultures that Rome had absorbed into its empire. From Egypt came the cult of the goddess Isis, from Persia Christianity.

The gods and goddesses of Olympus survived for a time but in altered form. During the third century, all that was vital in the traditional pagan cults was incorporated into a new philosophical scheme called "Neoplatonism" (based loosely on the much earlier though of the Greek philosopher Plato). Neoplatonism was the creation of the third century philosopher Plotinus, one of the most influential minds of the Romans imperial era. Plotinus taught the doctrine of one god, who was infinite, unknowable and unapproachable, except through a mystical experience. This deity was the ultimate source of everything, spiritual and physical. All existence was conceived of as a series of circles radiating outward from him, like concentric ripples in a pond, diminishing in excellent and significance as they grew more distant from their divine source. Human reason, which the Greeks had earlier exalted, now lost its fascination, for at the core of reality was a god that lay beyond reason's scope.

Plotinus and his followers regarded the multitudes of pagan gods and goddesses as crude but useful symbols, pointing to the overarching Neoplatonic god. Though poorly suited to the deepening mood of otherworldliness, the pagan cults acquired new life under under the canopy of Neoplatonic philosophy, which brought them into line with the trend toward mysticism and monotheism. The distinction between Jupiter and the new Eastern deities was steadily blurring.

During the third century conditions grew still worse. Invading tribes broke through the frontiers again and again, forcing cities to construct protective walls and threatening for a time to tear the Empire to pieces.  Imperial survival depended increasingly on a military defense, and the Roman legions, well aware of the fact made and unmade emperors. Roman armies repeatedly battled one another for control of the imperial office until (to  exaggerate only slightly) a man might be a general one day, emperor the next, and dead the third. Some twenty-odd emperors (depending on whom you count) reigned during the calamitous half-century between 235 and 285, and all but one were murdered, were killed in combat or died in captivity. With political anarchy came social and economics breakdown. The cost of living soared 1000% between A.D 256 and 280, and the increase in numbers of soldiers and administrators forced the government, in order to pay their salaries, to levy higher and higher taxes on its peasants and townspeople. Many fled their homes and jobs to escape the tax collector. The golden age had given way to what one third-century writer described as an "age of iron and rust".

The Empire was saved, though just barely, by a series of warrior-emperors of the late third and early fourth centuries. By tremendous military effort they threw back Germanic raiders and Persian armies, recovered the lost provinces and restored the old frontiers. They also took measures to arrest the social and economic decay and to reconstruct the Romans administration - on severely authoritarian lines. These measures have often been criticized, and with some justice. But they enabled the faltering empire to survive for nearly two more centuries in the West and for over a thousand years in the East.