The following account of the Church's general progress, from the close of the age of Heathen persecution to the doctrinal settlement effected by the Fourth Ecumenical Council, is based substantially on what I had formerly occasion to prepare for my pupils at Trinity College, Glenalmond. I would fain hope that it may to some extent enable the younger students of Ecclesiastical History, and general readers interested in the subject, to increase their knowledge of a period which is second in importance to the Apostolic age alone. The Church held on her way, not without spot or blemish, not without struggle and suffering, yet still beautiful and "terrible as an army with banners," victorious by the strength of her faith in one royal and world-redeeming Name. - Preface.