The Council of Nicaea and the creed
Bishops called by Constantine meet at Nicaea to settle whether Jesus is truly God, producing the core of the creed Catholics still recite every Sunday.
- The Early Church — Henry Chadwick, Penguin (1993)
- A History of Christianity: The First Three Thousand Years — Diarmaid MacCulloch, Allen Lane (2009)
- Catechism of the Catholic Church (2nd edition) — Holy See, Libreria Editrice Vaticana (1997)
- Leo XIV (official pages of the Holy See) — Holy See, Holy See
- The Orthodox Church — Timothy Ware, Penguin (1993)
In the early fourth century an Alexandrian priest named Arius taught that the Son of God was created by the Father and so was not fully God. The dispute split the eastern church, and in 325 Constantine summoned bishops, traditionally numbered at 318, to Nicaea, near his capital. The council declared that the Son is “of one substance” (homoousios) with the Father, and condemned Arius. The council also set a common way of calculating the date of Easter.
- The Early Church — Henry Chadwick, Penguin (1993)
- A History of Christianity: The First Three Thousand Years — Diarmaid MacCulloch, Allen Lane (2009)
The argument did not end there. Later councils at Constantinople (381), which expanded the creed and affirmed the divinity of the Holy Spirit, Ephesus (431), which called Mary Theotokos, “Mother of God”, and Chalcedon (451), which defined Christ as one person in two natures, divine and human, shaped the doctrine of the Trinity Catholics hold today. Chalcedon also caused the first lasting split, with churches in Egypt, Syria and Armenia that rejected it.
- A History of Christianity: The First Three Thousand Years — Diarmaid MacCulloch, Allen Lane (2009)
- Catechism of the Catholic Church (2nd edition) — Holy See, Libreria Editrice Vaticana (1997)
- Leo XIV (official pages of the Holy See) — Holy See, Holy See
Why it matters today
The Nicene Creed, "God from God, Light from Light, true God from true God", is said at Sunday Mass worldwide and is shared with Orthodox and most Protestant churches. In November 2025 Pope Leo XIV marked its 1,700th anniversary at the site of Nicaea, in today's İznik in Turkey, with the Orthodox Ecumenical Patriarch.
Orthodox Christians call themselves the church of the seven ecumenical councils, and Nicaea is the first. They recite the creed of Nicaea and Constantinople at every Divine Liturgy without the Western "filioque", and keep Constantine and his mother Helena as saints "equal to the apostles". Many still celebrate Easter by the Julian calendar, partly out of loyalty to the early church's reckoning.