To combat many heresies the Church knew that it had to make clear to the faithful that Jesus was both fully human and fully divine. The need to combat heresy also gave rise to the first ecumenical councils.
The word ecumenical comes from the Greek and means "of the whole world." These ecumenical councils of the Church stand out as expressions of the guidance of the Holy Spirit, and bring together the bishops of the whole world with the bishop of Rome, the pope to guide the Church in matters of faith and life in Jesus Christ.
In the early church many heresies emerged to challenge the Christian faith. Some of the most important works of the Father of the Church were written to defend the truths of the faith against heetics.
The Council of Nicea was founded by Emperor Constatine. They were started to prove that Jesus and God the Father were the same. They also wrote the Nicene Creed.
Emperer Theodosius called a council at Constantinople and said "the blessed trinity is the central mystery of Christian faith and life" and the Nicecne Creed was added at Constantinople. It is true what Theodosius said because we live our lives in our faith.