He made it the official religion of the Roman Empire.
Explanation:
Emperor Theodosius was the last ruling emperor of Rome before it collapses. He was mostly remembered for his act of legalizing Christianity as the official religion of his kingdom and making the power of the ruler beneath that of the church.
Even though Christianity had been legally accepted in Rome during the reign of Constantine, it wasn't necessarily made the official religion. The Council of Nicaea called by Emperor Constantine to specify the religious doctrinal belief system of the Christians, where many Christian priests and leaders attended. The resolution or outcome of this council came to be known as the Nicean Creed, which Emperor Theodosius adopted for the legal Christian belief doctrine for all Christians. He then made Christianity the official religion of Rome before he died, thus cementing the status of Christianity in the nation.
