When were the key beliefs of the Catholic church initially defined?

The core Roman Catholic beliefs were first defined in 325 A.D. The beliefs now known as the Nicene Creed or the Constantinopolitan Creed were defined in the Councils of Nicaea and Constantinople.

The Nicene Creed is the culmination of a meeting of church leaders that formally commenced on May 20, 325 A.D. After the council meeting, the church leaders took the key beliefs home, and they became widespread in the course of the next 60 years. During that time, some clarifications and additions were added to the creed. The Nicene Creed is recited every Sunday in liturgical churches as part of the liturgy.