Which Ecumenical Council is recognized for defining the doctrine of the Trinity?

The First Council of Nicaea, convened in 325 AD, is recognized for defining the doctrine of the Trinity, particularly in its formulation of the Nicene Creed. This council addressed the Arian controversy, which questioned the divinity of Jesus Christ and his relationship to God the Father. The bishops gathered at Nicaea affirmed that Jesus is "true God from true God," establishing that the Son is consubstantial with the Father, meaning they share the same divine essence. This was a pivotal moment in Christian theology as it clarified the nature of the relationships within the Trinity—Father, Son, and Holy Spirit—and set forth a foundational statement of belief that is still recited in various Christian liturgies today.

Other councils, such as the Council of Chalcedon and the Council of Ephesus, focused on different theological disputes, particularly concerning Christology and the nature of Christ. The Second Vatican Council, held in the 20th century, primarily dealt with issues of modernity within the Church and did not alter the fundamental doctrine of the Trinity as established at Nicaea. Thus, the First Council of Nicaea is rightly viewed as the council that laid the groundwork for Trinitarian doctrine in Christianity.