What was the primary purpose of the First Ecumenical Council, Nicaea I in 325 AD?

The primary purpose of the First Ecumenical Council, Nicaea I in 325 AD, was to address the Arian controversy and establish the Nicene Creed. This council was convened mainly in response to the teachings of Arius, a priest from Alexandria, who asserted that Jesus Christ was a created being and not co-eternal with God the Father. His views threatened the unity of the Christian faith by suggesting a hierarchy within the Trinity, which could undermine key theological principles.

The Nicene Creed, formulated during this council, was a statement of faith that affirmed the belief in the consubstantiality of the Son with the Father, clearly stating that Jesus is "true God from true God." This formal declaration was crucial as it sought to unify the Christian doctrine and counter the Arian beliefs that were gaining traction at the time.

Through this affirmation of faith, the council not only addressed the immediate theological concerns posed by Arianism but also set a precedent for defining orthodox Christian doctrine through future ecumenical councils. This focus on establishing a unified belief system is what makes the role of Nicaea I particularly significant in the history of Christian theology.