Ecumenical council

Reformed Christians accept four ecumenical councils.

Council of Nicaea (325 AD): Affirmed the full divinity of Jesus Christ and condemned Arianism, establishing the Nicene Creed and the doctrine of the Trinity (the Son is of the same essence as the Father).
Council of Constantinople (381 AD): Reaffirmed the Nicene Creed, affirming the full divinity of the Holy Spirit and clarifying the doctrine of the Trinity as equally one in essence and three in persons.
Council of Ephesus (431 AD): Affirmed the title "Theotokos" (God-bearer) for Mary, defending the truth of Christ's full humanity and divinity against Nestorianism, which sought to separate the two natures of Christ.
Council of Chalcedon (451 AD): Defined the doctrine of the hypostatic union, asserting that Jesus Christ is fully God and fully man, with two distinct natures united in one person, opposing both Monophysitism and Nestorianism.