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Why did Emperor Constantine call the First Council of Nicaea?

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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.

Emperor Constantine I convened the First Council of Nicaea in 325 CE primarily to resolve the theological controversy caused by Arianism within the Eastern church.12

Arianism, proposed by the Alexandrian presbyter Arius, asserted that Christ was a created being rather than divine and of the same substance as God the Father. This teaching challenged the unity of the church and the prevailing understanding of the Trinity. Constantine, hoping to restore harmony and unity among the body of believers, called for this general council to address these conflicting views and establish a clear, orthodox statement of faith.132

The council ultimately condemned Arianism as a heresy and affirmed the full divinity of Christ by adopting the term homoousios ("of one substance") in the Creed of Nicaea, thereby defining the official Christological position of the church.132