To remove all doubts as to the reality of his conversion, he wrote, while yet a catechumen, his celebrated work against the Pagans, in seven books (), which we still possess. The time when he wrote it, is not quite certain : some assign its composition to the years A. D. 297 and 298, but it is more probable that it was written in or shortly after the year A. D. 303, since it contains some allusions (as 4.36) to the persecution of the Christians by Diocletian, which commenced in that year. The work is a vindication of Christianity, and the author first refutes the charges of the Pagans against the Christian religion, especially the one which was then frequently brought against it, that the sufferings and calamities of the times were only the fruits of Christianity. He then proceeds to prove, with great learning, acuteness, and eloquence, that polytheism is irreconcilable with good sense and reason, and tends to demoralize mankind. In the sixth book he describes the superiority of the Christian religion; and the last contains a justification of the Christian views respecting sacrifices, and a comparison of the Christian notions of the Deity and divine things with those of the Pagans.

In writing this work, Arnobius was evidently animated by a genuine zeal to establish the truth of Christianity, but was free from the eccentricity and enthusiasm of Tertullian. His style is plain and lucid; though animated and sometimes rhetorical, it is yet not free from harsh and barbarous expressions : he treats of his subject with calmness and dignity, and is on the whole a pleasing writer, and superior to his contemporaries. As regards his knowledge of Christianity, it is difficult to form a decided opinion, for it was either his intention to set forth only the main doctrines of Christianity against the pagan mythology, or he possessed but a limited knowledge of the Christian religion. The latter is indeed the more probable, since he wrote his work when yet a catechumen. What he says in his second book about the nature and immortality of the soul, is not in accordance with Christian views, but with those of the Gnostics, and at a later time would have been regarded as heretical. The Old Testament seems to have been altogether unknown to him, and he slows no acquaintance with the New, except so far as the history of Christ is concerned. In regard to heathen antiquity, on the other hand, its religion and modes of worship, the work exhibits most extensive and minute learning, and is one of our best sources of information respecting the religions of antiquity. It is for this reason that Vossius calls him the Varro of the early Christian writers. The arrangement of his thoughts is philosophical, though not always sufficiently strict. Arnobius is a writer worthy to be studied not only by theologians, but also by philologers. He is not known to have written anything besides his book against the Gentiles; there are, however, some works whlich hae sosoetimes been ascribed to him, though they manifestly belong to a later writer or writers of the same name. (See the following article.)