The Third Ecumenical Council:  The Council of Ephesus 24Canon II.

If any provincial bishops were not present at the holy Synod and have joined or attempted to join the apostacy; or if, after subscribing the deposition of Nestorius, they went back into the assembly of apostates; these men, according to the decree of the holy Synod, are to be deposed from the priesthood and degraded from their rank.