Complex sentence: Christianity in the form of the Nicene Creed became the official religion of the empire in 380 via the Edict of Thessalonica issued in the name of three emperors-Gratian, Valentinian II and Theodosius I-with Theodosius clearly the driving force behind it.
Split into simpler sentences: Christianity in the form of the Nicene Creed became the official religion of the empire in 380. It was issued in the name of the three emperors-called the Edict of Thessalonica-by Gratian, Valentinian II and Theodosius, who was clearly the driving force behind it.