Perhaps the most lasting monument of Justinian's reign was his codification of Roman law. By this time it had become necessary to rewrite many of the laws as they had become obsolete since their last codification by Theodosius is 348. In an absolute monarchy the people ceased to be the source of the laws. It was now the monarch, by virtue of his office, that was responsible for putting into effect a new law, as well as the way in which it was interpreted and enforced.
Justinian also reformed the government and ordered a review of Roman law. This undertaking led to the publication of the Code of Justinian, a digest of Roman and church law, texts, and other instructional materials that became the foundation of modern Western law. Justinian also participated actively in the religious arguments of his day.
The heritage of Roman law represented an unbroken tradition that continued down to the time of Justinian. Preservation and renewal of the laws, Justinian felt, offered the possibility of emphasizing one of major roots of the empire's strength. This immense accomplishment far outlasted the Byzantine Empire and survived to form the basis of European jurisprudence.
On February 13, 528, Justinian appointed ten jurists to compile a new codification of the statute law. The ten-man commission appointed to the task of compiling the new code included two men of particular significance. In 534, a commission appointed by Justinian produced the Corpus Juris Civilis (The Body of Civil Law) which was widely accepted, in many parts of Europe and elsewhere, as the standard legal work until the middle of the 19th century.
Following this, any new legislation, when needed, was from that point onward issued in the form of "New Constitutions", known as "Novels". These dealt with such issues as ecclesiastical and public affairs, private law, and one very long Novel in particular constitutes a code of Christian marriage law.
Justinian's achievements in law were more long-lasting. Although several collections of imperial Roman legislation had been compiled in the past, by Justinian's reign even the most recent, the Theodosian Code (Codex Theodosianus), which had been issued in 438, was out-of-date. Accordingly in 528 Justinian established a commission of ten experts, including Tribonian, to prepare a new edition, which was completed in 534. The Code (Codex), as it was called, contains 4,562 laws from the reign of Hadrian (117–138) to 534.
By the eleventh century, succession to the Byzantine throne had degenerated into a power struggle between the civil and military aristocracies. On the other hand, the secular and theological universities flourished despite the political instability, and the emperors proved to be generous patrons of the arts. Basil I (867-886) and Leo VI (886-912) oversaw the collection and reform of the law codes.
Leo, the most prolific lawgiver since Justinian, sponsored the greatest collection of laws of the medieval Byzantine Empire, a work that would affect jurisprudence throughout Europe. Constantine VII Porphyrogenitus (912-959) excelled as a military leader, lover of books, promoter of an encyclopedia, and surveyor of the empire's provinces. At a time when scholarship in western Europe was almost nonexistent, Byzantine society featured a rich cultural life and widespread literacy among men and women of different classes.