Emperor Marcus Antoninus grants all free-born men of the Roman Empire full citizenship.

The Crisis of the Third Century shakes the foundation of the Roman Empire with civil war, external invasions, and economic collapse. The crisis began with the assassination of Roman Emperor Alexander Severus.

Origen's Hexapla is written as a six translation text of the Old Testament canon. Eusebius would eventually use this in his efforts to develop an accepted canon.

The first codification of Hebrew oral laws in the Jewish Mishna.

Arius, the eventual father of Arianism, is born in North Africa.

Emperor Valerian massacres Christians.

Constantine I is born in Serbia to a Roman general.

Eusebius of Caesarea, the “father of church history,” is born.

Diocletian becomes emperor of Rome and restores the empire from crisis by establishing an autocratic government. He quickly began work on the second phase of the Roman Empire often referred to as the Tetrarchy or Later Roman Empire.

Diocletian, realizing that the empire is much too big for one man to rule, divides the empire under two powers. He alone would rule the east and Maximian would rule the west.

Diocletian and Maximian set up a Caesar underneath them who would rule in their absence. In the east, under Diocletian, was Galerius. In the west, under Maximian, was Constantine.

Arius of Alexandria introduces arianism, denying the divinity of Christ.

Eusebius sees Constantine, along with Diocletian, in Palestine.