As it appears that the thread has concluded, I thought that I ought to say that the statement is from the Fourth Ecumenical Council, of Chalcedon, 451 A.D., (Act V). (It was, in turn, based on the Tomos of Saint Leo the Great, Pope of Rome.)
The statement is considered as authoritative/normative by the Eastern Orthodox, Roman Catholics, and most historic Protestants.
My own direct source for the statement was the Book of Common Prayer (Episcopal Church, USA, 1979), in the section, "Historical Documents of the Church."
