380 · Thessalonica (modern Thessaloniki), Greece
Edict of Thessalonica
Emperor Theodosius I declared Nicene Christianity the official religion of the Roman Empire, making Catholicism the state faith and outlawing other forms of Christianity and paganism.
On February 27, 380, Emperor Theodosius I issued the Edict of Thessalonica, declaring that all subjects of the Roman Empire should profess the faith of the bishops of Rome and Alexandria, meaning Nicene Trinitarian Christianity. It was the first time a Roman emperor had mandated a specific form of Christianity as the official state religion. The Edict of Milan in 313 had legalized Christianity. The Edict of Thessalonica went much further: it made one version of Christianity mandatory and branded everyone else, Arian Christians, pagans, and other heretical groups, as demented and insane, subject to punishment by the state. This was a dramatic shift. Christians who had been persecuted just decades earlier were now in a position to persecute others, and some of them did. Pagan temples were closed. Arian churches were confiscated. The old Roman tolerance for religious diversity, which had always been limited but real, was gone. The marriage of Catholic orthodoxy with imperial enforcement created a model that would dominate European politics for more than a thousand years. It also created a precedent that would trouble the Church's conscience: if it is wrong for the state to persecute Christians, is it not also wrong for Christians to use the state to persecute others?
Why it matters
Made Nicene Christianity the official religion of the Roman Empire, establishing the alliance of Church and state that would define European Christendom for over a millennium.
What Happened
Theodosius, a baptized Nicene Christian, issued a decree from Thessalonica requiring all Roman subjects to adopt the Trinitarian faith professed by the bishops of Rome and Alexandria. Arianism, paganism, and other heterodox beliefs were declared illegal. Within a year, Theodosius convened the First Council of Constantinople to reinforce Nicene orthodoxy. Pagan temples were gradually closed, and their property was confiscated. The Roman Empire, which had persecuted Christians for centuries, was now enforcing Christianity.
Why It Still Matters
The Edict of Thessalonica set the template for Christendom: one faith, enforced by the state. That model persisted in various forms until the Enlightenment, and its echoes are still felt in countries with established churches. Vatican II's declaration on religious freedom (Dignitatis Humanae) effectively reversed the logic of the Edict, teaching that faith must be free and coercion has no place in religion. Understanding this arc, from persecuted minority to state religion to defender of religious liberty, is essential for grasping how Catholic thinking on freedom of conscience has developed.