The Nicene Creed is a profession of the basic doctrines of Christianity as they are believed to have been revealed by Jesus and passed on by his Apostles. It is also meant as a standard to separate orthodox, or true, doctrine from heresy. Many Christian traditions other than Catholicism accept this creed.
Though many Christians believe that the contents of the Nicene Creed come from Jesus and His Apostles, the creed itself is the product of a historical development in which the church confronted many heresies concerning the faith. The creed was developed in part as a response against those heresies. It served as a means to make explicit those teachings that the church had always held, but which at the time were not sufficiently developed and defined.
An example is the section of the creed that states that Jesus is consubstantial, or of the same substance, as God the Father. This teaching comes from key passages in the New Testament, but had rarely been given a concise and precise formulation. In the fourth century A.D., the Arian heresy taught that Jesus was a lesser creature than God. The Council of Nicaea convened in 325 to condemn this heresy and affirm the apostolic teaching of Jesus' equality and consubstantiality with the Father.
The creed is, in fact, structured along the lines of the three persons of the Trinity. The first section is about the Father, the second about the Son and the third about the Holy Spirit. The section on the Holy Spirit also encompasses basic Christian beliefs about the church.