The Edict of Thessalonica (Greek: Έδικτο της Θεσσαλονίκης), issued on 27 February AD 380 by Theodosius I, made Nicene Christianity[note 1] the state church of the Roman Empire.
[5] This edict, addressed to the inhabitants of Constantinople whom Theodosius wished to pacify in order to make the city his imperial residence, constitutes the first known secular law which includes in its preamble a clear definition of what a Christian Roman ruler considers as religious orthodoxy, opening the way of repression against dissidents qualified as "heretics".
The council produced the original text of the Nicene Creed, which rejected the Arian confession and upheld that Christ is "true God" and "of one essence with the Father.
Cunctos populos, quos clementiae nostrae regit temperamentum, in tali volumus religione versari, quam divinum Petrum apostolum tradidisse Romanis religio usque ad nunc ab ipso insinuata declarat quamque pontificem Damasum sequi claret et Petrum Aleksandriae episcopum virum apostolicae sanctitatis, hoc est, ut secundum apostolicam disciplinam evangelicamque doctrinam patris et filii et spiritus sancti unam deitatem sub pari maiestate et sub pia trinitate credamus.
Hanc legem sequentes Christianorum catholicorum nomen iubemus amplecti, reliquos vero dementes vesanosque iudicantes haeretici dogmatis infamiam sustinere ‘nec conciliabula eorum ecclesiarum nomen accipere’, divina primum vindicta, post etiam motus nostri, quem ex caelesti arbitro sumpserimus, ultione plectendos.
We order the followers of this law to embrace the name of Catholic Christians; but as for the others, since, in our judgment they are foolish madmen, we decree that they shall be branded with the ignominious name of heretics, and shall not presume to give to their conventicles the name of churches.