5 Moreover, since this jubilee Year marks the sixteenth centenary of the Council of Nicaea, We commanded that event to be celebrated, and We have done so in the Vatican basilica. There is a special reason for this in that the Nicene Synod defined and proposed for Catholic belief the dogma of the Consubstantiality of the Onlybegotten with the Father, and added to the Creed the words "of whose kingdom there shall be no end," thereby affirming the kingly dignity of Christ.
Source: Quas Primas (Vatican.va)