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How did the Roman Empire transition from pagan polytheism to Christianity?

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Ruins of the Roman Temple of Diana in Evora, Portugal, featuring tall stone columns on a raised stone platform, with historic buildings in the background. — Roman religion Roman temple, known as the Temple of Diana, in Évora, Portugal.

The Roman Empire transitioned from pagan polytheism to Christianity through a series of significant developments. Initially, Christianity was a persecuted minority religion, with Romans viewing its followers with suspicion due to their rejection of traditional Roman gods and the imperial cult.12

A pivotal moment was the Edict of Milan in 313 CE, issued by Emperors Constantine I and Licinius. This proclamation established religious toleration for Christianity, ending systematic persecution and granting Christians legal rights. While it did not make Christianity the official religion, it allowed the faith to grow and gain influence.31

Constantine I played a crucial role by legalizing Christianity and providing financial support to the church, including sponsoring church-building projects and convening theological councils. The religion officially became the state religion of the Roman Empire in 380 CE under Emperor Theodosius I with the Edict of Thessalonica. This marked a profound shift, elevating Christianity above other religions and profoundly shaping Western civilization.431