III. The city of Constantinople, royal though it be, can never be raised to Apostolic rank.
Let the city of Constantinople have, as we desire, its high rank, and under the protection of God's right hand, long enjoy your clemency's rule. Yet things secular stand on a different basis from things divine: and there can be no sure building save on that rock which the Lord has laid for a foundation. He that covets what is not his due, loses what is his own. Let it be enough for Anatolius that by the aid of your piety and by my favour and approval he has obtained the bishopric of so great a city.
Let him not disdain a city which is royal, though he cannot make it an Apostolic See; and let him on no account hope that he can rise by doing injury to others. For the privileges of the churches determined by the canons of the holy Fathers, and fixed by the decrees of the Nicene Synod, cannot be overthrown by any unscrupulous act, nor disturbed by any innovation. And in the faithful execution of this task by the aid of Christ I am bound to display an unflinching devotion; for it is a charge entrusted to me, and it tends to my condemnation if the rules sanctioned by the Fathers and drawn up under the guidance of God's Spirit at the Synod of Nicæa for the government of the whole Church are violated with my connivance (which God forbid), and if the wishes of a single brother have more weight with me than the common good of the Lord's whole house.
Source: Letters (New Advent)