Julius Firmicus Maternus, author of De Errore Profanarum Religionum and Mathesis, is an important but oftentimes overlooked writer from the middle of the fourth century. He is known to us only from the two works which he left behind, the former being a Christian polemic against pagan religion and the latter, a work he wrote while still a pagan, being on the subject of astrology. It is his Christian work which is the topic of this thesis. The middle of the fourth century when Firmicus wrote his work, A.D. 346-350, was a time of religious change and struggle in the Roman Empire. Within Christianity there were still troubles over the issues which precipitated the Council of Nicea. Outside of the church, paganism, though on the defensive, was still strong. Legislation had been passed against the pagan cults but it was not being enforced. So, about A.D. 348, a Roman Senator, Julius Firmicus Maternus, wrote a letter Concerning the Error of Profane Religions to the Emperors Constans and Constantius. The first section of this work, chapters 1-17, presents the various gods of antiquity. Firmicus ridicules these by depicting the crimes and immorality of the gods, by showing that the pagan gods were nothing more than personified elements or processes of nature. His arguments are basically those found in previous Christian writers. In the last portion of this work, chapters 18-29, Firmicus relates the "passwords" used in the mystery cults for recognition and transmission of secret knowledge. Most of these "passwords," which are preserved for us in Greek, contain motifs or words which are similar to motifs and words found in Christianity; e.g., rock, bridegroom, eucharist, redemption from suffering. Firmicus sees these as attempts of Satan to trick mortal man and keep him from believing in Christ. This, too, is a motif found in previous Christian writers. However, in spite of Firmicus' Christian polemic, most scholars believe that Firmicus is very valuable and reliable as a source for the pagan beliefs of late antiquity. In addition to his value as a source of pagan religion, Firmicus is also very important to us for his statements concerning the relationship between church and state in the fourth century. In Firmicus, we find for the first time the articulation of the view that the Emperor is a servant of God whose purpose is to destroy paganism and all its visible forms. Firmicus also believes that forced conversion is beneficial to the individual. Although the pagans may now object to the forced used on them, after they are saved, they will thank the Emperor for forcing them to embrace Christianity. Firmicus' views take on added significance when one realizes that he was probably representative of popular Christianity. As a recent convert, Firmicus may well represent what the average, patriotic Christian layman of the fourth century believed should be the relationship between Christianity and the Roman Empire. This thesis was begun as an attempt to present to the English reading student of late antiquity a faithful translation of an important, but usually overlooked, writer from the fourth century. In addition, I have included a small introduction and helpful commentary; however, it is always best to let the writer speak for himself.