The sources of church history, the data on which we rely for our knowledge, are partly divine, partly human. For the history of the kingdom of God from the creation to the close of the apostolic age, we have the inspired writings of the Old and New Testaments. But after the death of the apostles we have only human authorities, which of course cannot claim to be infallible. These human sources are partly written, partly unwritten.
I. The written sources include:
(a) Official documents of ecclesiastical and civil authorities: acts of councils and synods, confessions of faith, liturgies, church laws, and the official letters of popes, patriarchs, bishops, and representative bodies.
(b) Private writings of personal actors in the history: the works of the church fathers, heretics, and heathen authors, for the first six centuries; of the missionaries, scholastic and mystic divines, for the middle age; and of the reformers and their opponents, for the sixteenth century. These documents are the richest mines for the historian. They give history in its birth and actual movement. But they must be carefully sifted and weighed; especially the controversial writings, where fact is generally more or less adulterated with party spirit, heretical and orthodox.
(c) Accounts of chroniclers and historians, whether friends or enemies, who were eye-witnesses of what they relate. The value of these depends, of course, on the capacity and credibility of the authors, to be determined by careful criticism. Subsequent historians can be counted among the direct or immediate sources only so far as they have drawn from reliable and contemporary documents, which have either been wholly or partially lost, like many of Eusebius authorities for the period before Constantine, or are inaccessible to historians generally, as are the papal regesta and other documents of the Vatican library.
(d) Inscriptions, especially those on tombs and catacombs, revealing the faith and hope of Christians in times of persecution. Among the ruins of Egypt and Babylonia whole libraries have been disentombed and deciphered, containing mythological and religious records, royal proclamations, historical, astronomical, and poetical compositions, revealing an extinct civilization and shedding light on some parts of Old Testament history.
II. The Unwritten sources are far less numerous: church edifices, works of sculpture and painting, and other monuments, religious customs and ceremonies, very important for the history of worship and ecclesiastical art, and significant of the spirit of their age.
The works of art are symbolical embodiments of the various types of Christianity. The plain symbols and crude sculptures of the catacombs correspond to the period of persecution; the basilicas to the Nicene age; the Byzantine churches to the genius of the Byzantine state-churchism; the Gothic cathedrals to the Romano-Germanic catholicism of the middle ages; the renaissance style to the revival of letters.
To come down to more recent times, the spirit of Romanism can be best appreciated amidst the dead and living monuments of Rome, Italy, and Spain. Lutheranism must be studied in Wittenberg, Northern Germany, and Scandinavia; Calvinism in Geneva, France, Holland, and Scotland; Anglicanism at Oxford, Cambridge, and London; Presbyterianism in Scotland and the United States; Congregationalism in England and New England. For in the mother countries of these denominations we generally find not only the largest printed and manuscript sources, but also the architectural, sculptural, sepulchral, and other monumental remains, the natural associations, oral traditions, and living representatives of the past, who, however they may have departed from the faith of their ancestors, still exhibit their national genius, social condition, habits, and customs -- often in a far more instructive manner than ponderous printed volumes.