His great apologetical work was the treatise undertaken at the special request of his friend Ambrosius, in answer to the attack of the heathen philosopher Celsus on the Christian religion, in a work which he entitled Λόγος ἀληθής, or _A True Discourse_. Origen states that he had heard that there were two individuals of this name, both of them Epicureans, the earlier of the two having lived in the time of Nero, and the other in the time of Adrian, or later.[31] Redepenning is of opinion that Celsus must have composed his work in the time of Marcus Aurelius (161-180 A.D.), on account of his supposed mention of the Marcionites (whose leader did not make his appearance at Rome before 142 A.D.), and of the Marcellians (followers of the Carpocratian Marcellina), a sect which was founded after the year 155 A.D. under Bishop Anicetus.[32] Origen believed his opponent to be an Epicurean, but to have adopted other doctrines than those of Epicurus, because he thought that by so doing he could assail Christianity to greater advantage.[33] The work which Origen composed in answer to the so-styled _True Discourse_ consists of eight books, and belongs to the latest years of his life. It has always been regarded as the great apologetic work of antiquity; and no one can peruse it without being struck by the multifarious reading, wonderful acuteness, and rare subtlety of mind which it displays. But the rule which Origen prescribed to himself, of not allowing a single objection of his opponent to remain unanswered, leads him into a minuteness of detail, and into numerous repetitions, which fatigue the reader, and detract from the interest and unity of the work. He himself confesses that he began it on one plan, and carried it out on another.[34] No doubt, had he lived to re-write and condense it, it would have been more worthy of his reputation. But with all its defects, it is a great work, and well deserves the notice of the students of Apologetics. The table of contents prefixed to the translation will convey a better idea of its nature than any description which our limits would permit us to give. DOGMATIC WORKS.