The historical figure associated with the codification of church law in the Byzantine Empire is Justinian. He is best known for his comprehensive legal reforms, particularly the creation of the "Corpus Juris Civilis," or the Body of Civil Law, which systematically compiled and organized existing legal texts and interpretations. This pivotal work not only influenced the development of ecclesiastical law but also served as a foundational document for subsequent legal systems in Europe.
Justinian’s reign marked a significant period of legal consolidation, articulating the relationship between the state and the church, and establishing a framework that would govern ecclesiastical matters. His efforts in codifying church law were instrumental in standardizing practices and doctrines within Byzantium, which had long-lasting implications for both the church and the state.
Other options, while significant in their own rights—like Constantine, who played a crucial role in the promotion of Christianity, or Charlemagne, who worked on church reforms in the context of his empire—do not specifically pertain to the formal codification of church law in the way Justinian's actions did. Theodosius's contributions were mainly in relation to the establishment of Christianity as the state religion, but he did not engage in the systematic codification efforts that characterized Justinian