The original Nicene Creed of 325

The original Nicene Creed was first adopted in 325 at the First Council of Nicaea, which was the Ecumenical Council. At that time, the text ended after the words "We believe in the Holy Spirit."
The Coptic Church has the tradition that the original creed was authored by Athanasius. F.J.A. Hort and Adolf Harnack argued that the Nicene creed was the local creed of Caesarea brought to the council by Eusebius of Caesarea. J.N.D. Kelly sees as its basis a baptismal creed of the Syro-Phoenician family, related to but not dependent of the creed cited by Cyril of Jerusalem and to the creed of Eusebius.
Soon after the Council of Nicaea, new formulas of faith were composed, most of them variations of the Nicene Symbol, to counter new phases of Arianism. The Catholic Encyclopedia identifies at least four before the Council of Sardica (341), where a new form was presented and inserted in the Acts of the Council, though it was not agreed on.

The Nicene Creed of 381

The second Ecumenical Council in 381 added the remainder of the text except for the words "and the son"; this is the version still used by Eastern Orthodox and Greek Catholic churches today.
The third Ecumenical Council reaffirmed the 381 version, and stated that no further changes could be made to it, nor could other creeds be adopted.