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How did Arianism challenge the belief that Jesus was fully divine?

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Christology The Transfiguration, the nature of Jesus as the Son of God being revealed to the apostles Peter, James, and John, mosaic icon, early 13th century; in the Louvre, Paris.

Arianism challenged the belief in the full divinity of Jesus by asserting that he was a created being rather than coeternal and consubstantial with God the Father. Proposed by the Alexandrian presbyter Arius, this Christological position was rooted in the premise that God is unique, immutable, and self-existent.123

Because Arius argued that the Godhead cannot be shared or communicated, he maintained that the Son—who is mutable and had a beginning—must be a creature brought into existence out of nothing by the will of the Father. Consequently, Arianism posited that the Son was of a different order of existence and could not possess direct knowledge of the Father.13

Opponents, such as St. Athanasius, argued that this view reduced Jesus to a demigod and undermined the Christian doctrine of redemption, which required the savior to be truly divine. The First Council of Nicaea ultimately rejected Arianism, affirming that the Son is homoousios ("of one substance") with the Father.132