Julius, an ecclesiastical writer, who lived about the middle of the fourth century. Nothing is known with certainty respecting his country, profession, or character, as we find no mention made of him in the writings of ancient authors. Some say, that he was by birth a Sicilian, and practiced in the forum as a barrister for some time, becoming a convert to Christianity when far advanced in years; which appears to derive considerable support from different passages in his writings. He was author of a treatise De errore profanarum religionum, which was dedicated to the emperors Constantius and Constans. This work must Firmicus must have been written between 340 and 350, in which Constans was slain by Magnentius. It is allowed to be a learned, able, and well written performance, in which the reasonableness of the Christian religion is strongly contrasted with the absurdity and immorality of the gentile creed. It must not be dissembled, however, that he sometimes betrays such a spirit of intolerance as is wholly incompatible with the genius of the Christian religion, which breathes nothing but benevolence towards the whole human race. The arguments employed by him in its defense are disgraced by an exhortation to the civil power to propagate it by force of arms, and to crush the advocates of error by severe edicts. This work was first published at Strasbourg in 1562, at Heidelberg in 1559, and at Paris in 1670.

The greater part of critics ascribe to him a work entitled Astronomicon, seu de Matheis, lib. viii. In it he treats of the power and influence of the stars, agreeably to the doctrine of the Egyptians and Babylonians, blending a considerable degree of mathematical knowledge with the unmeaning jargon of judicial astrology. Those who imagine that so good a man as Firmicus could not have been the author of such an absurd performance, should remember that it was probably composed prior to his conversion, when such absurdities would constitute a part of his creed.