Thomasius argued that the Corpus Iuris Civilis, compiled under Emperor Justinian in the sixth century A.D., was an important example of the growth of clerical power. Roman law was widely used in legal arguments in Thomasius's time and although legal historians have tended to focus on its decreasing importance in this period, numerous contemporary jurists continued to defend its usefulness and equitability. In 1717, the Dutch jurist Wilhelm Best gave a public oration “On the Equity of Roman Law and the Pleasure of Studying It.” Thomasius's colleague at the University of Halle, Samuel Stryk, claimed that almost all European kingdoms esteemed the Corpus Iuris “on the most equitable grounds on which jurisprudence rests” and drew on it “whenever the particular laws of the kingdom were insufficient.”Leibniz, in a letter to Hermann Conring, said that the rules of strict natural justice harmonized perfectly with the maxims in the last title of Justinian's Digest, De regulis juris.

Thomasius's more critical attitude toward Roman Law is usually seen either as an example of an early “Germanism” in legal theory, that is, the consciousness of a national legal culture, which prohibits the use of the foreign Corpus Iuris Civilis in German courts, or as a reflection of Thomasius's natural law theory, in which he is said to have rejected Roman law whenever it conflicted with the precepts of natural law. However, an important but neglected reason for Thomasius's criticisms of Justinian's Corpus Iuris is that he believed its compilation coincided with the corruption of Christianity in late antiquity and was affected by it. At the beginning of his essay on the title De Summa Trinitate from Justinian's Codex, he commented that the Corpus Iuris was full of the “fruits of Papist religion.” The essay then continued with an account of the decline of the church from the charity and piety of the early Christians, which set in soon after the apostolic age, once Christians had begun to debate doctrines, as if faith consisted in an intellectualist orthodoxy and not in purity of heart.