Fr. Thomas Hopko: The dogmatic definitions (dogma means official teaching) and the canon laws of the Ecumenical Councils are understood to be inspired by God and to be expressive of His will for men. Thus, they are essential sources of Orthodox Christian doctrine.
Nicea II 7th Ecumenical Council 787
The pattern for those who have received the sacerdotal dignity is found in the testimonies and instructions laid down in the canonical constitutions, which we receiving with a glad mind, sing unto the Lord God in the words of the God-inspired David, saying: I have had as great delight in the way of your testimonies as in all manner of riches. You have commanded righteousness as your testimonies for ever. Grant me understanding and I shall live. Now if the word of prophesy bids us keep the testimonies of God forever and to live by them, it is evident that they must abide unshaken and without change. Therefore Moses, the prophet of God, speaks after this manner: To them nothing is to be added, and from them nothing is to be taken away. And the divine Apostle glorying in them cries out, which things the angels desire to look into, and, if an angel preach to you anything besides that which you have received, let him be anathema. Seeing these things are so, being thus well-testified unto us, we rejoice over them as he that has found great spoil, and press to our bosom with gladness the divine canons, holding fast all the precepts of the same, complete and without change, whether they have been set forth by the holy trumpets of the Spirit, the renowned Apostles, or by the Six Ecumenical Councils, or by Councils locally assembled for promulgating the decrees of the said Ecumenical Councils, or by our holy Fathers. For all these, being illumined by the same Spirit, defined such things as were expedient. Accordingly those whom they placed under anathema, we likewise anathematize; those whom they deposed, we also depose; those whom they excommunicated, we also excommunicate; and those whom they delivered over to punishment, we subject to the same penalty. And now let your conversation be without covetousness, cries out Paul the divine Apostle, who was caught up into the third heaven and heard unspeakable words.
Ancient Epitome: We gladly embrace the Divine Canons, viz.: those of the Holy Apostles, of the Six Ecumenical Synods, as also of the local synods and of our Holy Fathers, as inspired by one and the same Holy Spirit. Whom they anathematize we also anathematize; whom they depose, we depose; whom they cut off, we cut off; and whom they subject to penalties, we also so subject. (Canon 1)