Why did Justinian I make laws about religion and religious groups?

Justinian I enacted extensive legislation regarding religion because he viewed the Byzantine Empire as a terrestrial counterpart to the heavenly polity, where church and state were indissolubly linked. As the emperor, he considered himself the viceregent of Christ, making it his duty to promote the good government of the church and uphold orthodox teaching.12

His laws aimed to regulate almost every aspect of Christian life, from baptism and the sacraments to the conduct of the laity and clergy, to ensure the spiritual health of the empire. Justinian believed that failure to maintain religious order—such as through blasphemy, sacrilege, or the persistence of non-Christian practices—could invite divine wrath in the form of famine, pestilence, or natural disasters.3

Consequently, he used his legal authority to:
* Enforce Orthodoxy: He sought to convert subjects to his specific brand of Christianity and outlawed Paganism, forbidding pagans, heretics, and Samaritans from teaching.12
* Standardize Practice: He regulated the administration of churches and monasteries to improve the quality of the clergy.3
* Manage Dissent: He attempted to reconcile different Christian factions, though these efforts often led to further political and religious tension.13