300 – 349
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