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What was the Arian controversy about the nature of Jesus Christ?

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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.

The Arian controversy was a major 4th-century theological dispute concerning the nature of Jesus Christ and his relationship to God the Father.12

The conflict originated with Arius, an Alexandrian presbyter who argued that God the Father was unique, self-existent, and immutable. Based on this, Arius contended that the Son (Jesus) could not be the same as the Father; instead, he believed the Son was a created being, brought into existence out of nothing, and therefore subordinate and of a different order of existence than God.13

Opponents, led by St. Athanasius, argued that this view reduced Christ to a demigod and undermined the Christian doctrine of redemption, asserting that only one who was truly God could reconcile humanity to the Godhead.14

The First Council of Nicaea in 325 condemned Arianism as a heresy, affirming that the Son was homoousion—of the same substance—as the Father. The controversy persisted for decades through various political and ecclesiastical shifts until the First Council of Constantinople in 381, which solidified the Nicene Creed and effectively proscribed Arianism.15