The knowledge of Church history is very important in understanding the Catholic faith. Notably, John Henry Cardinal Newman was converted to Catholicism after embarking on a monumental study on Church history for his An Essay on the Development of Christian Doctrine in 1845. There are some non-Catholic Christians who detest talking about Church history, without twisting some facts, because then they would discover that only the Catholic Church could trace herself back to the Church founded by Christ. This is why we say that Pope Benedict XVI is the 265th successor to Saint Peter. Some of them argue by way of defense that the Catholic Church later became an apostate church, which contradicts the Lord’s promise in Matt. 16:18, or that the true Church of Christ was hidden until a designated time in history when it emerged, which negates the Lord’s command in Matt. 28:19. They refuse to accept the fact that Martin Luther may have had the right motive, but had the wrong method in his efforts to reform the Church because his reformation did not take place within, but without the Church when he established his own Church, unlike Saint Francis before him. The knowledge of Church history is also very useful in defending the Catholic faith.
One of the significant moments in the history of the Church came in the fourth century where several watershed events occurred. These events were the Edict of Milan, the Council of Nicaea, the conversion of Saint Augustine, and the formulation of the Biblical Canon.
The Edict of Milan was the big break the Christians were waiting for after reeling from its first three centuries of birth pains plagued by harassment and heresy, and capped by the Great Persecution of Diocletian. The legal acceptance of Christianity by the Roman Empire under Constantine the Great gave the Church the necessary peace to expand and grow, and gave way to the development of a dynamic relationship between Church and State where the Pope became both a spiritual and secular leader. They also had their disadvantages conducive to the abuses wrought during the Crusades and the Inquisition, and the corruption of the Papacy that reached its height during the Renaissance and that gave birth, to some extent, to the Western Schism, the Reformation, the Enlightenment and the Jeffersonian separation of Church and State.
The First Ecumenical Council of Nicaea was the first ecumenical council of the Catholic Church and it was bound to set the standard for the resolution of dogmatic debates binding on the universal Church, although the first council that embarked on such effort was the local Council of Jerusalem (Acts 15) that regulated the relationship between the Christians and the Jews. Nonetheless, the Nicene Council was the first to define doctrine for the whole Church to follow although the nature of the Holy Spirit would be definitively defined in the second ecumenical council of the Church in the same century or the First Ecumenical Council of Constantinople. The said two ecumenical councils gave us the creed drawn up from the deposit of the Apostolic Faith that curbed the spread of a pernicious heresy even beyond the lifetimes of Arius and Athanasius.
The conversion of Saint Augustine gave us the most prolific and brilliant theologian and philosopher among the Church Fathers or Doctors whose speculations have been authoritative for both Catholics and Protestants, and have been the subject of debates for centuries (Pelagianism, Calvinism, Molinism etc.) especially those dealing with divine grace and human freedom as they impinge on the doctrine of original sin and predestination, the power of God and the dignity of the human person. His influence on Western thought is significant, such as his doctrine on the just war, the way of dealing with heretics and the relationship between faith and science. He was the first to attempt a synthesis of Greek philosophy and Christian theology through his Christianization of Platonism and to formulate ideas on monasticism. He is considered the intellectual master of many illustrious Church personalities, like Saint Thomas Aquinas and Pope Benedict XVI.
The formulation of the Biblical Canon became a necessity in the light of the fact that there had been thousands of Gospels, Acts, Epistles and Revelations and versions thereof attributed to the Apostles and their contemporaries in circulation that had to be sifted to separate the canonical from the apocryphal. The Canon was confirmed by the local Synods of Rome and Hippo and the Council Carthage, which was later affirmed by the Ecumenical Council of Trent in the 16th century. Hence, the New Testament canon was completed during this century and it was a definitive list until Luther came along.
The fourth century, among the Christian centuries, had witnessed several watershed events that had changed the life of the Church.