The fourth century began with the greatest persecution ever waged against the early Church, that of the emperor Diocletian. The longest list of early martyrs comes from this period (303-306).
After Diocletian's abdication, a power struggle developed among the imperial leaders. In 312, Constantine engaged in battle with his main contender for the western throne, Maxentius. Before the battle of the Milvian bridge near Rome, Constantine had a vision, perhaps in a dream. He saw the Cross or Labarum (Chi Rho: XP) of Christ with the words, "In this sign, conquer." He placed the Christian symbol on his troop's tunics and weapons, and they won the battle.
In 313, Constantine met Licinius, the Eastern ruler of the empire, in Milan. Together they issued an edict giving freedom to Christians to practice their faith in the empire. Before Constantine died he built a city in the ancient site of Byzantium for his new imperial capital. He named the city after himself - Constantinople. Constantine himself was baptized only on his deathbed in 337. Together with his mother, Helen, who recovered the True Cross of Christ in Jerusalem, Constantine is recognized as a saint of the Church. Christianity became the official religion of the empire in 380 by decree of the emperor Theodosius.
During Constantine's time, the Church recovered its property and was free from external persecution. Inner troubles immediately arose, however, to disturb the peace. First, there was the Donatist Schism in North Africa. This was a schism between those who supported a certain Donatus to be the bishop of Carthage, and those who supported the regularly elected bishop a man whom the Donatists opposed because of his weakness in the time of the persecution. Instead of forcing the Church to solve its own problems, Constantine intervened in the controversy. First, he sided with the Donatists, then he sided with their opposers, using imperial power to enforce his decisions. The schism resulted in the ultimate destruction of the once glorious Church in North Africa, and established the precedent of imperial intervention in Church affairs.
The Arian controversy then arose. Arius, an Alexandrian priest, taught that the Divine Logos, the Word of God who became man - Jesus Christ - is not the divine Son of God. He was merely a creature like everything else created out of nothing by God. According to Arius, God is not the uncreated Holy Trinity. God is the Father, the Creator, alone. God the Father created His Logos or Word or Son as the first and greatest of His creatures. This Logos, Who may be called divine only in a manner of speaking is God's instrument for the salvation of the world, being born as the man Jesus. Thus Jesus Christ is not the uncreated, divine Son of God having exactly the same uncreated divinity as God the Father. He is a creature, as is the Holy Spirit. God is not the Holy Trinity.
The First Ecumenical Council
The controversy raised by the teaching of the Arians was brought to the decision of the whole Church at the Council which Constantine called in Nicea in 325. This council, known as the First Ecumenical Council, decreed that the Logos, Word and Son of God is uncreated and divine. He is begotten - that is, born or generated - from the Father, and not made or created by Him. He is of one essence with the Father (homoousios). He is True God of True God, the Word of God by Whom all things were made. It is this uncreated, only-begotten divine Son of God Who became man from the Virgin Mary as Jesus Christ the Messiah of Israel and the Savior of the world.
The Second Ecumenical Council
The decision of the Nicene Council was not universally accepted in the Church for a long time. The controversy raged for many decades. Numerous councils were held in different places which formulated various statements of faith. The Arian party gained imperial support and the defenders of the Nicene faith were greatly persecuted. The troubles persisted until 381 when, at a council in Constantinople, known now as the Second Ecumenical Council, the original decision of Nicea was reaffirmed and the divinity of the Holy Spirit was proclaimed. The combined statement of these two councils comprises the Symbol of Faith, the Creed of the Orthodox Church.
The Fathers of the Church
The great defenders of Nicene Orthodoxy were Saint Athanasius the Great, bishop of Alexandria (d.373) and the Cappadocian bishops, Saint Basil the Great (d.379), his brother Saint Gregory of Nyssa (d.394), and their friend Saint Gregory Nazianzus the Theologian (d.389). These fathers of the Church taught and explained the true Christian faith, suffering greatly for their defense of the central doctrine of Orthodox Christianity, that God is the Most Holy Trinity: three uncreated and divine persons of the Father, the Son, and the Holy Spirit, in one and the same uncreated, divine nature.
The Councils of the Church
The Council of Nicea also made a number of canons concerning the order and discipline of the Church. These canons confirmed the primacy of the Church of Rome in the West, Alexandria in Africa, and Antioch in the East (Canon 6), and the recognition of the dignity of the Church in Jerusalem (Canon 7). The council also made the rules for determining the date of the annual celebration of Easter. The council prohibited the practice of penitential kneeling at the Church’s Sunday liturgy (Canon 20).
The Council of Constantinople also produced canons, one of which stated that "the bishop of Constantinople shall have the prerogative of honor after the bishop of Rome because Constantinople is the New Rome." (Canon 3)
The fourth century witnessed a number of liturgical developments. During this time, the eucharistic prayers of the divine liturgies, named after Saint Basil the Great and Saint John Chrysostom (d.407) were substantially formulated. The catechetical sermons of Saint John Chrysostom together with those of Saint Cyril of Jerusalem (d.386) show that the sacraments of Baptism and Chrismation were being celebrated in the fourth century almost exactly as they are done in the Orthodox Church today.
By this time, the 40 Day Lent and the Easter Feast were well established. The Nativity of Christ was separated from the feast of Epiphany or Theophany, thus becoming a separate feast of the Church to offset the pagan festival of the Sun which was celebrated on the twenty-fifth of De- cember.
The fourth century also saw the flourishing of monastic life in Egypt - led by Saint Anthony the Great (d.356) - in Syria, and in the West. Among the monastic saints of this period were Paul of Thebes, Pachomius, Hilarion, Sabbas, Macarius of Egypt, Epiphanius of Cyprus, and Ephraim of Syria. Among the monastic saints in the West were Jerome, John Cassian, and Martin of Tours. The famous bishop saints of the fourth century were Saint Nicholas of Myra in Lycia, Saint Spyridon Trimunthys, and Saint Ambrose of Milan.