300 The church council of Elvira, Spain, prohibits intermarriage between Jews and Christians, also forbidding them to eat together.
301 The Kingdom of Armenia is the first nation to make Christianity its official religion.
303 Emperor Diocletian orders the persecution of Christians.
312 Constantine defeats Maxentius at Battle of Milvian Bridge and becomes the ruler of the western Roman Empire. He believes the Christian God has guided him to victory.
313 Edict of Milan issues by Constantine I (Constantine the Great, the first Christian Roman emperor – he converted on his death bed in 337), allowing Christians to practice their faith in the Roman Empire.
325 Constantine the Great summons the Council of Nicaea, which establishes that God the Father and God the Son (Jesus Christ) are of the same essence. Constantine introduces Sunday as a holy day in a new 7-day week. He also introduced movable (Easter) and immovable feasts (Christmas).
326 Constantine the Great and his mother Helena start a perdio of building churches in Palestine to mark the places considered holy to Christianity, including the Church of the Holy Sepulchre in Jerusalem and the Church of the Nativity in Bethlehem.
330 Founding of Constantinople (which became Istanbul in 1900), built above the ancient site of Byzantium, which became the centre of Christianity.
333 Constantine decrees that Christians of Jewish heritage break all ties with Judaism or be executed.
335 The Church of the Holy Sepulchre is consecrated on the site of what is believed to be Christ’s tomb.
337 Constantine the Great converts to Christianity on his deathbed, according to Bishop Eusebius.
341 Ethiopians introduced to Coptic Christianity
350 Christianity reaches Ireland.
359 Under the leader of the Sanhedrin, Nasi Hillel II fix the calendar according to a standardized system of calculations (as used today). Previously, the calendar was based on eye witness of sightings of the new moon.
363 Constantine’s grandson becomes emperor, becoming known as Julian the Apostate. H rescinds the law that forbids marriage between Christians and Jews and rescinds the law that bans Jews from entering Jerusalem; he also abolishes privileges that have been bestowed upon the Christian clergy.
367 Emperor Julian killed while fighting the Sassanid Empire.
372 Mahayana Buddhism introduced to Koguryo (northern Korea).
378 Valens, the Christian emperor of the eastern half of the Roman empire, is defeated by Christian Germans, Goths, at Adrianople.
380 Co-emperors Gratian and Theodosius decress that the doctrine of the Trinity is the official state religion.
383 Shapur III becomes king of the Sassanid empire.
384 Buddhism introduced to the royal families of Paekche (southern Korea) and Silla (central Korea).
388 Shapur III, the Sassanid king, lifts the persecutions of Christians.
390 Jerome’s Latin Vulgate manuscripts published, containg all 80 books of the Scriptures (39 Old Testestament, 27 New Testament, 14 Apocrypha).
391 One of the Seven Wonders of the World, the library of Alexandria, is destroyed by fire.
395 Division of the Roman Empire, when Emperor Theodosius I dies. (His two sons appointed successors: 17-year-old Arcadius given rule over the east; 10-year-old Honorius ruling the west, but ruling from Milan instead Rome.) Augustine is named bishop of Hippo (in North Africa).
399 St. Augustine of Hippo writes his Confessions.
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