Answer:
Theodosius in 390 CE.
Step-by-step explanation:
In, 390 CE, Theodosius who was the king of Roman empire made Christianity the official religion of the state. In 300 CE, the Christianity grew rapidly among the people of Romans and large number of Romans accept this religion. In 313 CE, the king of the roman empire also accepted Christianity as a religion so with the passage of time it becomes the official religion of the roman empire in the reign of Theodosius.