The first ecumenical council was the First Council of Nicaea, which was held in Nicaea (modern-day Iznik, Turkey) in 325 AD. It was convened by the Roman Emperor Constantine I to address the issue of Arianism, a controversial teaching that denied the divinity of Jesus Christ.

At the council, the bishops declared that Jesus Christ was of the same substance (homoousios) as God the Father, and condemned the Arian teaching that Jesus was a created being. This doctrine of the Trinity, which asserts that there is one God in three persons (the Father, the Son, and the Holy Spirit), became the orthodox Christian teaching and is still held by most Christians today.

The council also established a uniform date for celebrating Easter and issued other canons related to church discipline, such as the ordination of bishops and the treatment of heretics.

The decisions made at the First Council of Nicaea were highly influential in shaping Christian theology and doctrine for centuries to come.
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