Humility

Quote from Letter 5,8 to Felix, bishop of Serdica. October 594.

You yourself have no doubt as to how obedience and reverence ought to be shown to your superiors and to your inferiors also. In this matter, it is most advantageous if humility that deserves applause can uphold what the force of discipline imposes, without anyone's pressure.
And so, it has come to our attention that your Fraternity does not deign to obey our brother John, bishop of Prima Justiniana, according to custom, and are unwillling to add your signature in the normal way either to his decree or to the repley that he sent to us.
But if this is so, we are most upset.
For you are clearly showing proof of arrogance on your part. And the words of a divine warning state loudly how especially culpable this is, where a priest is concerned.

Cited from: The Letters of Gregory the Great, trans. John R.C. Martyn (Toronto: PIMS, 2004), II: 328.