Second Council of Nicaea, Canon I

That the sacred Canons are in all things to be observed.

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 thy testimonies as in all manner of riches.”  “Thou hast commanded righteousness as thy 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, speaketh 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 ye have received, let him be anathema.”  Seeing these things are so, being thus well-testified unto us, we rejoice over them as he that hath 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,” crieth out Paul the divine Apostle, who was caught up into the third heaven and heard unspeakable words.

Translation: Henry R. Percival, The Seven Ecumenical Councils, NPNF II.14 (1900), via CCEL.