Defining the faith: the ecumenical councils and the creeds

As the church grew, it had to say precisely what it believed, and it did so at a series of ecumenical ("worldwide") councils whose decisions Catholic, Orthodox, and most Protestant churches still accept. The First Council of Nicaea (325), called by Constantine, answered the teaching of Arius, that the Son was a created being, less than the Father, by confessing the Son "of one substance (homoousios) with the Father": fully God, not a creature. Its statement, expanded at the First Council of Constantinople (381), is the Nicene Creed still recited across the traditions, which confesses one God in three persons, Father, Son, and Holy Spirit. The Council of Ephesus (431) affirmed the unity of Christ's person; the Council of Chalcedon (451) gave the classic definition that Christ is one person "in two natures," fully God and fully man, "without confusion, change, division, or separation." Alongside the creeds, the church settled its canon, which books belong in the Bible, over the same centuries, ratified in regional councils (Hippo 393, Carthage 397) and in the writings of figures like Athanasius and Jerome (the canon's history is treated in this reference's apologetics and book-intro material). Not every church accepted every council: the Oriental Orthodox churches (Coptic, Armenian, Ethiopian, Syriac) parted after Chalcedon over how to describe Christ's natures, and remain distinct to this day, an early reminder that "the church" was never perfectly single. But the Nicene faith forged in these councils became the shared trunk from which every later branch grows.