Teaching of the Church
The teaching of the Church of Christ must be consitent and non-contradictory over time. When things became unclear on specific doctrine, Ecumenical Councils (i.e. universal council involving the entire Christian Church)
were called. During the first millennium there were seven councils.
Significantly, most of the councils specifically referenced and reaffirmed the decisions of previous councils:
...in order that the divinely inspired tradition of the catholic church should receive confirmation by a public decree. So having made investigation with all accuracy
and having taken counsel, setting for our aim the truth, we neither diminish nor augment, but simply guard intact all that pertains to the catholic church. Thus, following the six holy universal synods... [Seventh Ecumenical Council, Nicea AD 787]
The Nicene Creed, finalized at the Second Ecumenical Council (AD 381), clearly affirmed the traditional doctrinal elements of the Father, Son and Holy Spirit (Trinity). the Third Ecumenical Council of Ephesis (431) forbade any alteration of the Creed or the wording: ...it is unlawful for any man to bring forward, or to write, or to compose a different Faith as a rival to that established by the holy Fathers assembled with the Holy Ghost in Nicæa"
After the Schism of AD 1054 councils have been held in the East and West as needed. The Roman Catholic Church teaches that their councils are 'ecumenical' because
the Pope is present.
While the Roman Catholic Churchs has grown numerically since separation,
there was basically an equal number of
Christians in the Orthodox and Roman Catholic halves of the world in the 11th century.
Since the Ecumenical Councils always united the vast majority, in some cases all, of the bishops from around the known world in their official teachings and judgements,
how can we regard any councils after the 7th as Ecumenical? Even secular meetings of government require a 'quorum' (minimum number of authorized participants) to conduct official business.
This is also the view of the Orthodox Church.
Additional Reference