The emperor Julian made a short-lived attempt to revive traditional and Hellenistic religion and to affirm the special status of Judaism, but in 380 under Theodosius I, Nicene Christianity became the official state religion of the Roman Empire. Pleas for religious tolerance from traditionalists such as the senator Symmachus (d. 402) were rejected. Christianity became increasingly popular. Heretics as well as non-Christians were subject to exclusion from public life or persecution, but Rome's original religious hierarchy and many aspects of its ritual influenced Christian forms, and many pre-Christian beliefs and practices survived in Christian festivals and local traditions.
Try to answer this question if possible (otherwise reply "unanswerable"): What was proclaimed the state religion under Theodosius I?
Nicene Christianity