This article records tradition as it has been passed down and reported. Its sources are not yet part of the atlas's verified catalogue.
In February 380 CE, the co-emperors Theodosius I, Gratian and Valentinian II issued a decree from Thessalonica requiring every subject of the Roman Empire to hold the Nicene Christian faith. The edict named other Christian positions, above all Arianism, as heresy and authorised punitive measures against anyone who held them, becoming, by the general account, the first Roman secular law to define religious orthodoxy and put the machinery of the state behind punishing doctrinal dissent.
The step was decisive rather than incremental. The Edict of Milan in 313 had granted Christianity toleration alongside every other religion in the empire; Thessalonica went a full stage further, converting that toleration into compulsion and naming a single doctrinal standard, Nicene orthodoxy, as the only lawful form of the faith. Arian Christians, who had held real influence at court under earlier emperors, now stood on the wrong side of the law themselves.
What survives of the edict comes down through the Theodosian Code, the compiled record of imperial law from which its text and context are known. What this changed for Christianity's spread was total: within a single generation of Milan's grant of bare toleration, the faith had become not merely legal but enforced, the empire's sole sanctioned religion, with the state's own authority now standing behind one specific doctrinal reading of it rather than behind Christianity in general.