4th century AD

Constantine's toleration of Christianity, formalized after his victory at the Milvian Bridge in 312 and confirmed at Milan the following year, changes what kind of century this is for Christian intellectual life more than any single doctrinal development within it. Within barely a decade the emperor is convening bishops at Nicaea in 325 to settle, by imperial-backed council, questions about Christ's relationship to God the Father that individual theologians had previously argued out independently and inconclusively. Eusebius, present at Nicaea, writes the Ecclesiastical History, a project begun earlier in the century and repeatedly revised into the Constantinian period, the earliest surviving sustained attempt to narrate Christianity's own institutional past now that doing so is no longer dangerous. Over the following decades the Cappadocian Fathers, Basil of Caesarea and Gregory of Nazianzus among them, and in Egypt the tradition Athanasius represents, give the doctrine settled in outline at Nicaea its full technical vocabulary in Greek, theological systematization every bit as rigorous, in its own terms, as anything Ptolemy or Galen produced two centuries earlier in their fields.

Two thousand miles south, a king converts to the same religion through a process his own coinage and inscriptions document with unusual directness. Ezana of Aksum, ruling from a kingdom on the Red Sea trade route whose armies campaign into the Nile valley around Meroe as the Kushite kingdom collapses during his reign, has his own royal inscriptions altered within that reign, crediting his military victories first to the traditional Aksumite gods and, in later inscriptions from the same reign, explicitly to the Christian god instead. Later Christian tradition, recorded by Rufinus, connects this shift to Frumentius, a Christian brought to Aksum's court through Red Sea merchant networks and later consecrated bishop by Alexandria's church; Aksum's conversion, on the evidence available, is a story of Red Sea commerce and ecclesiastical contact with Alexandria as much as it is a story of one king's own carved voice, and it proceeds on its own timeline, connected to Red Sea commerce, Christian merchant communities, Frumentius and the Alexandrian church rather than to the specific proceedings at Nicaea in the same general years far to the north.

Judaism undergoes its own major institutional consolidation this century, on a track independent of Christianity's imperial backing and considerably more precarious. The Mishnah, the codified oral law completed under Judah ha-Nasi's editorship around 220, has by this century become the base text a new generation of scholars in Roman Palestine argue over case by case, producing what becomes known as the Jerusalem Talmud across the second half of the fourth century, its redaction left incomplete, likely interrupted by the deteriorating conditions for Jewish communities under an increasingly Christian Roman administration in exactly the same province where Christian doctrine is simultaneously being formalized with full state backing. Genesis Rabbah, a verse-by-verse homiletical commentary treating Genesis as worthy of sustained interpretation in its own right, begins taking shape in the same milieu toward the century's end.

Rome's political structure cannot hold the shape Constantine gave it back together indefinitely. The catastrophic defeat and death of the emperor Valens at Adrianople in 378, at the hands of Gothic forces the empire had itself admitted as refugees fleeing the Huns, exposes the western frontier's fragility in a way no single battle had before; by 395, at Theodosius I's death, the empire divides permanently between his two sons into eastern and western halves that never again rule as one. Augustine writes the Confessions in these same final years of the century, an autobiographical examination of his own conversion, without clear earlier precedent in the same sustained form, that turns inward just as the empire around him is visibly fracturing outward. Far to the east, India's Gupta dynasty, founded in 320, is entering a genuine golden age under royal patronage that produces Kalidasa's court poetry and drama, a reminder that this century's crises are only some of Eurasia's story and not all of it. In northern China, the Jin dynasty's collapse after 316 opens the fragmented Sixteen Kingdoms period, non-Chinese steppe dynasties ruling the north while a diminished Jin court survives in the south, a political fracture running on its own schedule but rhyming, again, with Rome's.