Anchor Bible Dictionary entry for Celsus.
Gary T. Burke, "Celsus (Person)," The Anchor Yale Bible Dictionary, ed. David Noel Freedman, 6 vols. (Doubleday: Yale University Press, 1992), 1:879-881
A. The True Doctrine
A.1. The Trustworthiness of Origen's Quotations.
Although earlier scholarship was mixed on the reliability of Origen's preservation of the original TD, more recent studies have concluded in favor of Origen's care and general objectivity in his quotations from the TD. He preserved the larger part of the TD in some form, most of it in direct quotations. These citations were presented with a remarkable degree of exactness and absence of alteration for apologetic purposes. Further, in almost every instance, the original order of the TD is reflected in Origen's quotations.
A.2. Date.
More than a century ago Keim (1873) argued so convincingly that the TD was written in the year 178 c.e. that his dating became the scholarly consensus. Certain recent studies, however, have exposed major Haws in Keim's interpretation of key passages and have, in effect, reopened the date question. If, as they should be, Keim's arguments are rejected, the TD can be dated no more precisely than the last third of the 2d century.