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How did Theodosius I make Christianity the official religion of the Roman Empire?

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Mosaic: Christianity — Christianity Christ as Ruler, with the Apostles and Evangelists (represented by the beasts). The female figures are believed to be either Santa Pudenziana and Santa Práxedes or symbols of the Jewish and Gentile churches. Mosaic in the apse of Santa Pudenziana basilica, Rome, 401–417 ce.

Emperor Theodosius I established Christianity as the official religion of the Roman Empire primarily through the issuance of the Edict of Thessalonica on February 28, 380.123

Driven by both political and religious motives to achieve unity of faith, Theodosius mandated that all his subjects adhere to the Nicene Creed, specifically the belief in the consubstantiality of the Father, Son, and Holy Spirit. This edict officially designated those who followed this creed as Catholic Christians, marking the first time this term appeared in an imperial document.1

To further solidify this orthodoxy, Theodosius took the following steps:
* Ecclesiastical Sanction: He convened the First Council of Constantinople in 381, which formally adopted the Nicene Creed and clarified the theological formula for the empire.124
* Suppression of Alternatives: He actively suppressed Arianism and other non-Nicene Christian groups.14
* Prohibition of Paganism: He continued his efforts to establish religious hegemony by issuing a decree in 392 that prohibited paganism and traditional polytheistic practices.5

These actions elevated Christianity above all other religions, transforming it from a tolerated faith into the official state religion of the Roman Empire.23