The key in my mind seems to be mutual acceptance of the faith statements of the Ecumenical Councils. This is because one of the main ideas in Orthodoxy is that Ecumenical Councils are either infallible when confirmed or at least of a maximum high level of authority, like Scripture.
Were there times in the past when some churches did not accept all the faith statements of Ecumenical Councils?
I know there were times when churches disagreed with some ritual things endorsed at Councils: The Roman Church did not agree with the Apostles' Canons' demand for rebaptising those with heretic baptisms, even though the Apostles' Canons were endorsed by an Ecumenical Council. But that is more of a ritual matter.
Canon 95 of the Council in Trullo allows for the reception of converts from schismatic and heretical groups by profession of faith, Chrismation or Baptism. Recognized by the 7th Ecumenical Council, Nicaea II in 787, the Council in Trullo has greater authority than the Apostolic Canons. Besides, what was considered an heretic at the time of the Apostles would be someone like a Gnostic or other sect that is not Christian. St. Basil defines an heretic as someone who worships different God than Christians. In the U.S. most Eastern Orthodox jurisdictions receive those Baptized "in the Name of the Father and of the Son and of the Holy Spirit" through Chrismation. We have to be very specific because of the influence of feminist theology which rejects Biblical language for God, and uses "Creator, Redeemer and Sanctifier," of whatever is politically correct this week instead of "Father, Son and Holy Spirit." Oriental Orthodox would be received by a simple profession of faith. Moscow also received Roman Catholics by a profession of faith.
Fr. John W. Morris