Short answers, pulled from the story.
The Church Fathers were ancient Christian theologians and writers who established the intellectual and doctrinal foundations of Christianity. They worked during the Patristic Era, which spans approximately from the late 1st to the mid-8th centuries, with the period flourishing particularly during the 4th and 5th centuries.
The First Epistle of Clement, written around 96 AD, is the earliest extant letter from a Church Father. Clement of Rome wrote it to call the Christians of Corinth to harmony and order. Some early canons of the New Testament listed it as canonical, and the historian Ibn Khaldun still counted it among New Testament writings as late as the 14th century.
The Cappadocian Fathers, Basil the Great, Gregory of Nyssa, and Gregory of Nazianzus, made major contributions to the definition of the Trinity, which was finalised at the First Council of Constantinople in 381. They formulated the phrase three hypostases in one homoousia, acknowledging a distinction between Father and Son while insisting on their essential unity, and worked to bring semi-Arian Christians back to orthodox teaching.
Tertullian is credited with introducing the Latin word trinitas to the Christian vocabulary and with formulating the phrase tres Personae, una Substantia (three Persons, one Substance). He also coined the terms vetus testamentum and novum testamentum for what became the Old and New Testaments. He was the first Latin author to call Christianity the vera religio in his Apologeticus.
Isaac of Nineveh, a 7th-century Syriac bishop and theologian, is recognised as a saint by the Church of the East, the Catholic Church, the Eastern Orthodox Church, and the Oriental Orthodox Churches, making him the last saint chronologically recognised by every apostolic church. His feast day falls on the 28th of January.
The Catholic Church holds that the Patristic Era has passed, with John of Damascus and Bernard of Clairvaux among those considered the last Church Fathers. The Eastern Orthodox Church does not consider the age of the Church Fathers to be over, and includes later influential writers up to the present day, guided by the consensus of the Holy Fathers rather than a fixed historical boundary.