The book presupposes the use of a text-book of Church history, such as those by Cheetham, Kurtz, Moeller, Funk, or Duchesne, and a history of doctrine, such as those of Seeberg, Bethune-Baker, Fisher, or Tixeront. Readings in more elaborate treatises, special monographs, and secular history may well be left to the direction of the instructor. The translations, with a few exceptions which are noted, are referred for the sake of convenience to the _Patrology_ of Migne or Mansi’s _Concilia_. Although use has been freely made of the aid offered by existing translations, especially those of the _Ante-Nicene_ and _Post-Nicene Fathers_, yet all translations have been revised in accordance with the best critical texts available. The aim in the revision has been accuracy and closeness to the original without too gross violation of the English idiom, and with exactness in the rendering of ecclesiastical and theological technical terms. Originality is hardly to be expected in such a work as this. An author may not be conscious of any attempt to make his selection of texts illustrate or support any particular phase of Christian belief or ecclesiastical polity, and his one aim may be to treat the matter objectively and to render his book useful to all, yet he ought not to flatter himself that in either respect he has been entirely successful. In ecclesiastical history, no more than in any other branch of history, is it possible for an author who is really absorbed in his work to eliminate completely the personal equation. He should be glad to be informed of any instance in which he may have unwittingly failed in impartiality, that when occasion presented he might correct it. The day has gone by in which ecclesiastical history can not be treated save as a branch of polemical theology or as an apologetic for any particular phase of Christian belief or practice. It has at last become possible to teach the history of the Christian Church, for many centuries the greatest institution of Western Europe, in colleges and universities in conjunction with other historical courses. This volume has been prepared at the suggestion of the American Society of Church History, and valuable suggestions have been gained from the discussions of that society. To Professor W. W. Rockwell, of Union Theological Seminary, New York, Professor F. A. Christie, of Meadville Theological School, the late Professor Samuel Macauley Jackson, of New York, and Professor Ephraim Emerton, of Harvard University, I have also been indebted for advice. The first two named were members with me of a committee on a _Source-Book for Church History_ appointed several years ago by the American Society of Church History.