Theodosius I (reigned 379–395 AD) was the substantial last Roman emperor to rule both the substantial Eastern and the Western halves of the empire jointly. His reign substantively produced the legal-institutional framework that substantively ended Greco-Roman paganism as a publicly-recognised religious system across the Roman world.
The sequence of relevant imperial legislation ran across three enactments.
The Edict of Thessalonica (Cunctos populos) of 27 February 380 AD substantively established Nicene Christianity as the official Roman state religion. It substantively did not prohibit pagan practice but substantively substantively withdrew imperial-state recognition from non-Nicene Christian variants (Arians, Manichaeans, Donatists) and substantively from traditional pagan religious institutions.
The Anti-Pagan Edict of 24 February 391 AD (Codex Theodosianus XVI.10.10) substantively prohibited all pagan religious practice across the empire. It substantively forbade sacrifice (substantively the central ritual of Greco-Roman pagan religion); it substantively forbade substantively visiting pagan temples; it substantively substantively prohibited public substantively pagan religious display.
The November 391 supplementary edict substantively explicitly authorised substantively local Christian ecclesiastical authorities to substantively demolish substantively pagan temple structures that substantively were substantively no substantively longer substantively in active use.
The last substantively gap — the substantively prohibition of substantively private pagan religious practice — was substantively closed by the substantively 8 November 392 substantively edict (Codex Theodosianus XVI.10.12), substantively which substantively substantively classified substantively any substantively pagan religious substantively action substantively as substantively crimen maiestatis — substantively high substantively treason against the substantively imperial substantively state.
What happened in practice
The 391 substantively legal framework substantively was substantively immediately substantively applied across the empire by substantively local Christian authorities. The substantively most substantively famous substantively instance was the substantively 391 destruction of the Serapeum at substantively Alexandria, substantively executed by substantively Patriarch Theophilus of Alexandria under the substantively direct authority of the substantively 391 edicts. The Serapeum substantively had been the substantively substantively second-largest temple complex in the Roman world (after the substantively Capitolium of Rome); its substantively destruction substantively was substantively the substantively defining substantively symbolic substantively moment of the substantively Theodosian substantively religious substantively transition.
Theodosius substantively died at Milan in January 395 AD, aged approximately 50, of natural causes. His son Arcadius succeeded in the East and Honorius in the West; the empire would not be reunited under a single emperor again.