He condemned other Christian creeds such as Arianism as heresies of "foolish madmen," and authorized their punishment.
It is 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 for repression against dissidents who qualified as "heretics". The Edict of Thessalonica was subsequently incorporated into Book XVI of the Theodosian Code and was the milestone of the official Christianization of the Roman Empire.
Ironically & amusingly, this decree came into play in 2021 when the responsa ad dubia regarding Traditionis Custodes were issued. As you undoubtedly know, it was forbidden to advertise the times of TLMs in Parish bulletins (talk about micromanagement overkill!). One inventive Priest got around the prohibition by advertising a 'Mass for Citizens of Rome'!
So, remember, we can all say, with St Paul, 'Civis Romanus sum' (Acts of the Apostles 22:25)!