The Communion of Faith

Letter 1,16 - to Severus, bishop of Aquileia. January 591.

"(For) not to know the truth involves less of an aberration than not to remain in the same truth once it is known, and what is committed by one who has gone astray is one thing, and what is perpetrated through knowledge is another. And indeed, if we had rejoiced over how far you had been incorporated, long ago, into the unity of the Church, we are most upset that you are now so abundantly disassociated from Catholic society.
For this reason, as soon as the bearer of this letter arrives, following the command of the most Christian and serene Lordship of all, we wish you to come to the tomb of Saint Peter the apostle with your followers, so that with God's support, in a special synod,a decision may be made about that doubt which is prevalent among you all."

Cited from: The Letters of Gregory the Great, trans. John R.C. Martyn (Toronto: PIMS, 2004), I: 131.