Julius Firmicus Maternus

Julius Firmicus Maternus was a Sicilian of senatorial rank who practiced law at Rome in the reign of Constantine. He is one of the very few writers of the ancient world who left major books on both sides of a conversion, and the pair of them can be read against each other.

Around 334 to 337, while still a pagan, he wrote the Matheseos libri VIII, eight books on astrology, dedicated to a provincial governor named Lollianus Mavortius. It is the fullest handbook of Roman astrology to survive: the meaning of the houses, the aspects of the planets, the fates written into a chart at birth. He wrote it, he says, after giving up the law because he was sick of the lying and the money.

Sometime after that he became a Christian. Around 346 to 350 he wrote a second book to the emperors Constantius II and Constans, De errore profanarum religionum, On the Error of the Profane Religions.

It is a strange and valuable book. Firmicus goes through the mystery cults one by one, the rites of Isis and Osiris in Egypt, Attis and the Great Mother, Adonis, Persephone, Mithras, and describes what happens in them: the passwords, the images, the mourning and the sudden joy, the words spoken over the initiate. Much of what modern historians know about these ceremonies is known because Firmicus wrote it down in order to attack it. He argues that the cults are the deification of natural forces and of dead men, that their priests deceive on purpose, and that a few of their rites are counterfeits of Christian ones planted by the devil.

Then he does something uglier. He appeals directly to the emperors to destroy the temples, seize their treasures, and stamp the cults out by force, quoting the harshest passages of Deuteronomy at them and telling the rulers that they will be judged if they leave the work half done. The book is one of the earliest Christian calls for state coercion in religion, written by a man who had himself been on the other side, and later Christian writers who wanted persuasion rather than compulsion did not follow him.

Only one manuscript of De errore survived, at the Vatican, and it has a gap in the middle. Nothing is recorded of his death; he is generally reckoned to have died around 350 or soon after. The astrological handbook, meanwhile, went on being copied for centuries by people who had no idea its author had ended up a Christian.

Sources: Firmicus Maternus, De errore profanarum religionum and Matheseos libri VIII; Philip Schaff, History of the Christian Church; the 1913 Catholic Encyclopedia.