Fathers of the Church are saintly writers of the first centuries of the Christian era, whom the Catholic Church acknowledges as witnesses of her faith.  To be numbered among the Fathers of the Church, four qualities are required of write.  First, you must have lived when the Church was in her youth; hence St. Gregory the Great (d. 604) is generally regarded as the last father in the West, St. John Damascene (d.754), in the East. 

    Secondly, he must have lived a saintly life.  Thirdly, his writings must not only be free from heresies, but also excel in the explanation and defense of Catholic doctrine.

    Lastly, his writings must bear the seal of the Churches approval thou the majority of the Fathers were bishops, yet this is not true of all of them.  St. Jerome was a simple priest to the end of his days, St. Ephraem a deacon, St. Justin a layman.

    All Fathers have not been proclaimed doctors of the Church.  In matters of faith and moral, the consent of the Fathers has always been held in high esteem by the Church.  What they unanimously teach to be of faith, is of faith; what they unanimously reject as heretical, is heretical.

    Even the logical conclusions which they unanimously draw from the articles of faith, furnish us with a certain theological argument.  Their authority is due not only to the facts that they were saints or bishops or eminent scholars and lived at a time when Christ's revelation was still fresh in the minds of men, but primarily to the approbation of the Church.  But Christ said of the Apostles, "He that hearth you, hearth me," the Church says in manner of the Fathers.   They are the mouthpiece of the infallible teaching of the Church, and the Church acknowledges them as witnesses of her own faith.  Hence, when anathematizing new heresies or defining new dogmas, the Councils appeal to the consent of the Fathers.  The Council of Ephesus (in 431) declared in its first session that it would define nothing save what had been held unanimously by the ancient and holy Fathers.  This approbation of the Church gives added authority even to the Fathers, considered singly thou in varying degrees period.

    A general approbation gives to a saintly writer of the first centuries implies that his doctrine in general is orthodox and worthy of recommendation.  Sometimes, however, a certain Father's doctrine receives a special approbation as being exceptionally solid; such is St. Augustine's fundamental doctrine on grace.

    Lastly, the highest degree of ecclesiastical approbation is reached when the Church takes the very doctrine of a Father and embodies it in her own official pronouncements, as in the case of the St. Cyril of Alexandria, whose twelve anathematisms against Nestorius were adopted by the Councils of Ephesus (431).  -C.E.; Agius, Tradition and the Church

    The word Father is used in the New Testament to mean a teacher of spiritual things, by whose means the soul of man is born again into the likeness of Christ: "For if you have ten thousand instructors in Christ, yet not many fathers. For in Christ Jesus, by the gospel, I have begotten you. Wherefore I beseech you, be ye followers of me, as I also am of Christ" (1 Corinthians 4:15, 16; cf. Galatians 4:19). The first teachers of Christianity seem to be collectively spoken of as "the Fathers" (2 Peter 3:4).

    Thus St. Irenaeus defines that a teacher is a father, and a disciple is a son (iv, 41,2), and so says Clement of Alexandria (Strom., I, i, 1). A bishop is emphatically a "father in Christ", both because it was he, in early times, who baptized all his flock, and because he is the chief teacher of his church. But he is also regarded by the early Fathers, such as Hegesippus, Irenaeus, and Tertullian as the recipient of the tradition of his predecessors in the see, and consequently as the witness and representative of the faith of his Church before Catholicity and the world. Hence the expression "the Fathers" comes naturally to be applied to the holy bishops of a preceding age, whether of the last generation or further back, since they are the parents at whose knee the Church of today was taught her belief. It is also applicable in an eminent way to bishops sitting in council, "the Fathers of Nicaea", "the Fathers of Trent". Thus Fathers have learnt from Fathers, and in the last resort from the Apostles, who are sometimes called Fathers in this sense: "They are your Fathers", says St. Leo, of the Princes of the Apostles, speaking to the Romans; St. Hilary of Arles calls them sancti patres; Clement of Alexandria says that his teachers, from Greece, Ionia, Coele-Syria, Egypt, the Orient, Assyria, Palestine, respectively, had handed on to him the tradition of blessed teaching from Peter, and James, and John, and Paul, receiving it "as son from father".

    It follows that, as our own Fathers are the predecessors who have taught us, so the Fathers of the whole Church are especially the earlier teachers, who instructed her in the teaching of the Apostles, during her infancy and first growth. It is difficult to define the first age of the Church, or the age of the Fathers. It is a common habit to stop the study of the early Church at the Council of Chalcedon in 451. "The Fathers" must undoubtedly include, in the West, St. Gregory the Great (d. 604), and in the East, St. John Damascene (d. about 754). It is frequently said that St. Bernard (d. 1153) was the last of the Fathers, and Migne's "Patrologia Latina" extends to Innocent III, halting only on the verge of the thirteenth century, while his "Patrologia Graeca" goes as far as the Council of Florence (1438-9). These limits are evidently too wide, It will be best to consider that the great merit of St. Bernard as a writer lies in his resemblance in style and matter to the greatest among the Fathers, in spite of the difference of period. St. Isidore of Seville (d. 636) and the Venerable Bede (d. 735) are to be classed among the Fathers, but they may be said to have been born out of due time, as St. Theodore the Studite was in the East.