There can be no great objection to the appearance of a text-book on the history of dogma at the present time. We now know in what direction we have to work; but we still want a history of Christian theological ideas in their relation to contemporary philosophy. Above all, we have not got an exact knowledge of the Hellenistic philosophical terminologies in their development up to the fourth century. I have keenly felt this want, which can only be remedied by well-directed common labour. I have made a plentiful use of the controversial treatise of Celsus against Christianity, of which little use has hitherto been made for the history of dogma. On the other hand, except in a few cases, I have deemed it inadmissible to adduce parallel passages, easy to be got, from Philo, Seneca, Plutarch, Epictetus, Marcus Aurelius, Porphyry, etc.; for only a comparison strictly carried out would have been of value here. I have been able neither to borrow such from others, nor to furnish it myself. Yet I have ventured to submit my work, because, in my opinion, it is possible to prove the dependence of dogma on the Greek spirit, without being compelled to enter into a discussion of all the details. The Publishers of the Encyclopaedia Britannica have allowed me to print here, in a form but slightly altered, the articles on Neoplatonism and Manichaeism which I wrote for their work, and for this I beg to thank them. It is now eighty-three years since my grandfather, Gustav Ewers, edited in German the excellent manual on the earliest history of dogma by Muenter, and thereby got his name associated with the history of the founding of the new study. May the work of the grandson be found not unworthy of the clear and disciplined mind which presided over the beginnings of the young science. _Giessen_, 1st August, 1885. AUTHOR'S PREFACE TO THE SECOND EDITION.