First Council of Nicaea

Convened by Emperor Constantine, the First Council of Nicaea was a defining moment in early Christian theology. The primary crisis was Arianism, a heresy taught by the priest Arius, which claimed that Jesus Christ was a created being and not co-eternal with God the Father. The Council definitively condemned Arianism, declaring that the Son is 'consubstantial' (homoousios) with the Father—true God from true God. The resulting Nicene Creed (later expanded at Constantinople) remains the universal profession of faith for Catholics, Orthodox, and most Protestants to this day.