Philip Schaff in the context of Patristic period

A Select Library of the Nicene and Post-Nicene Fathers of the Christian Church, usually known as the Nicene and Post-Nicene Fathers (NPNF), is a set of books containing translations of early Christian writings into English. It was published between 1886 and 1900.

Unlike the Ante-Nicene Fathers which was produced by using earlier translations of the Ante-Nicene Christian Library (ANCL), the Nicene and Post-Nicene Fathers was printed simultaneously in Europe and in America, by T. & T. Clark, by Christian Literature Company and other American editors. The translations were in the main revised versions of those made for the Oxford Movement Library of the Fathers series. T. & T. Clark was surely convinced by the commercial success of the cheaper American version/revision of the ANCL, although of lesser quality on some minor points. The Swiss-born, German-educated Philip Schaff was commissioned to supervise the first series of the NPNF. He was joined by the British Henry Wace for the second series.

Patristics, also known as patrology, is a branch of theological studies focused on the writings and teachings of the Church Fathers, between the first and the eighth centuries AD. Scholars analyze texts from both orthodox and heretical authors. Patristics emerged as a distinct discipline in the 19th century, supported by critical editions like Patrologia Latina and Patrologia Graeca. The field employs textual analysis, archaeology, and historical criticism to analyze early Christianity's doctrinal, cultural, and intellectual development, incorporating traditions beyond Greek and Latin, such as Syriac, Coptic, Armenian, and Ethiopian.

Formal principle and material principle are two categories in Christian theology to identify and distinguish the authoritative source of theology (formal principle) from the theology itself, especially the central doctrine of that theology (material principle), of a religion, religious movement, tradition, body, denomination, or organization. A formal principle tends to be texts or revered leaders of the religion; a material principle is its central teaching. Paul Tillich believes the identification and application of this pair of categories in theological thinking to have originated in the 19th century.

As early as 1845 the Protestant theologian and historian Philip Schaff discussed them in his The Principle of Protestantism. They were utilized by the Lutheran scholar F. E. Mayer in his The Religious Bodies of America in order to facilitate a comparative study of the faith and practice of Christian denominations in the United States. This is also treated in a theological pamphlet entitled Gospel and Scripture by the Commission on Theology and Church Relations of the Lutheran Church—Missouri Synod.