Under which Roman emperor did Christianity become the official state religion of the Roman Empire?

Theodosius I, also known as Theodosius the Great, was Roman Emperor from AD 379 to AD 395. Nicene Christianity became the state church of the Roman Empire with the Edict of Thessalonica in 380 CE, when he made it the Empire's sole authorized religion. The Eastern Orthodox Church, Oriental Orthodoxy, and the Catholic Church each claim to be the historical continuation of this church in its original form, but do not identify with it in the caesaropapist form that it took later. Unlike Constantine I, who with the Edict of Milan of 313 CE had established tolerance for Christianity without placing it above other religions, Theodosius established a single Christian doctrine, which he specified as that professed by Pope Damasus I of Rome and Pope Peter II of Alexandria, as the state's official religion.