The Catechism in Questions & Answers

33. Which are the most important of the early church creeds?

The two most important early church creeds are the Apostolic Creed (“Apostolicum”) and the Creed of Nicaea-Constantinople.
The basic features of the Apostolicum were compiled in the second century and lightly supplemented in the fourth century. The Creed of Nicaea-Constantinople is the result of the Council of Nicaea (in the year 325 AD) and the Council of Constantinople (in the year 381 AD). The main purpose of this creed was to enshrine the profession of the trinity of God.

A council is an assembly of senior religious dignitaries, who come together to discuss important matters of faith.