The First Council of Constantinople was a large meeting of Christian priests. It happened in the year 381. Emperor Theodosius I called the meeting in Constantinople.
At the meeting, the church adopted the Nicene Creed. The Nicene Creed is not the same as the Creed of Nicaea.
At the meeting, the church also decided on the Trinitarian Doctrine, which is about the relationship between God, Jesus and the Holy Spirit.