The Third Ecumenical Council:  The Council of Ephesus 29Canon VII.

When these things had been read, the holy Synod decreed that it is unlawful for any man to bring forward, or to write, or to compose a different (eteran) Faith as a rival to that established by the holy Fathers assembled with the Holy Ghost in Nicaea.

But those who shall dare to compose a different faith, or to introduce or offer it to persons desiring to turn to the acknowledgment of the truth, whether from Heathenism or from Judaism, or from any heresy whatsoever, shall be deposed, if they be bishops or clergymen; bishops from the episcopate and clergymen from the clergy; and if they be laymen, they shall be anathematized.

And in like manner, if any, whether bishops, clergymen, or laymen, should be discovered to hold or teach the doctrines contained in the Exposition introduced by the Presbyter Charisius concerning the Incarnation of the Only-Begotten Son of God, or the abominable and profane doctrines of Nestorius, which are subjoined, they shall be subjected to the sentence of this holy and ecumenical Synod. So that, if it be a bishop, he shall be removed from his bishopric and degraded; if it be a clergyman, he shall likewise be stricken from the clergy; and if it be a layman, he shall be anathematized, as has been afore said.