In the past, the Church Fathers were regarded as authoritative, and more restrictive definitions were used which sought to limit the list to authors treated as such. However, the definition has widened as scholars of patristics, the study of the Church Fathers, have expanded their scope.
In the Roman Catholic Church, they are also collectively called the "Eight Doctors of the Church", and in the Eastern Church, three of them (Basil the Great, Gregory of Nazianzus and John Chrysostom) are honored as the "Three Holy Hierarchs".
The earliest Church Fathers, (within two generations of the Twelve Apostles of Christ) are usually called the Apostolic Fathers since tradition describes them as having been taught by the twelve. Important Apostolic Fathers include Clement of Rome, Ignatius of Antioch, Polycarp of Smyrna, and Papias of Hierapolis. In addition, the Didache and Shepherd of Hermas are usually placed among the writings of the Apostolic Fathers although their authors are unknown; like the works of Clement, Ignatius and Polycarp, they were first written in Koine Greek.
His epistle, 1 Clement (c. 96), was copied and widely read in the Early Church. Clement calls on the Christians of Corinth to maintain harmony and order. It is the earliest Christian epistle aside from the New Testament.
Ignatius of Antioch (also known as Theophorus) (c. 35–110) was the third bishop or Patriarch of Antioch and a student of the Apostle John. En route to his martyrdom in Rome, Ignatius wrote a series of letters which have been preserved. Important topics addressed in these letters include ecclesiology, the sacraments, the role of bishops, and the Incarnation of Christ. He is the second after Clement to mention Paul's epistles.
Polycarp of Smyrna (c. 69 – c. 155) was a Christian bishop of Smyrna (now İzmir in Turkey). It is recorded that he had been a disciple of "John." The options for this John are John, the son of Zebedee, traditionally viewed as the author of the Gospel of John, or John the Presbyter. Traditional advocates follow Eusebius in insisting that the apostolic connection of Polycarp was with John the Evangelist, and that he was the author of the Gospel of John, and thus the Apostle John.