Nicene and Post-Nicene Fathers, Ser. II, Vol. XIV:
The Fourth Ecumenical Council.  The Council of Chalcedon.: Canon XVIII

Canon XVIII.

The crime of conspiracy or banding together is utterly prohibited even by the secular law, and much more ought it to be forbidden in the Church of God.  Therefore, if any, whether clergymen or monks, should be detected in conspiring or banding together, or hatching plots against their bishops or fellow-clergy, they shall by all means be deposed from their own rank.

Notes.

Ancient Epitome of Canon XVIII.

Clerics and Monks, if they shall have dared to hold conventicles and to conspire against the bishop, shall be cast out of their rank.

Bright.

In order to appreciate this canon, we must consider the case of Ibas bishop of Edessa.  He had been attached to the Nestorians, but after the reunion between Cyril and John of Antioch had re-entered into communion with Cyril on the ground that Cyril had explained his anathemas (Mansi, vii., 240), or, as he wrote to Maris (in a letter famous as one of the “Three Chapters”) that God had “softened the Egyptian’s heart” (ib., 248).  Four of his priests (Samuel, Cyrus, Maras, and Eulegius), stimulated, says Fleury (xxvij. 19) by Uranius bishop of Himeria, accused Ibas of Nestorianism before his patriarch Domnus of Antioch, who held a synod, but, as Samuel and Cyrus failed to appear, pronounced them defaulters and set aside the case (Mansi, vii. 217).  They went up to Constantinople, and persuaded Theodosius and archbishop Flavian to appoint a commission for inquiring into the matter.  Two sessions, so to speak, were held by the three prelates thus appointed, one at Berytus the other at Tyre.  At Berytus, according to the extant minutes (Mansi, vii., 212 ff.), five new accusers joined the original four, and charges were brought which affected the moral character of Ibas as well as his orthodoxy.  The charge of having used a “blasphemous” speech implying that Christ was but a man deified, was rebutted by a statement signed by some sixty clerics of Edessa, who according to the accusers, had been present when Ibas uttered it.  At Tyre the episcopal judges succeeded in making peace, and accusers and accused partook of the communion together (ib., vii., 209).  The sequence of these proceedings cannot be thoroughly ascertained, but Hefele (sect. 169) agrees with Tillemont (xv., 474 et seqq.) in dating the trial at Berytus slightly earlier than that at Tyre, and assigning both to the February of 448 or 449.  Fleury inverts this order, and thinks that, “notwithstanding the reconciliation” at Tyre, the four accusers renewed their prosecution of Ibas (xxvij. 20); but he has to suppose two applications on their part to Theodosius and Flavian, which seems improbable.  “The Council is believed,” says Tillemont (xv., 698), “to have had this case in mind when drawing up the present canon:”  and one can hardly help thinking that, on a spot within sight of Constantinople, they must have recalled the protracted sufferings which malignant plotters had inflicted on St. Chrysostom.

This canon is found in part in the Corpus Juris Canonici, Gratian’s Decretum, Pars II., Causa XI., Quæst. I., canons xxj. and xxiij.