Gregory to Quiricus, Bishop, and the other Catholic bishops in Hiberia .
Since to charity nothing is afar off, let those who are divided in place be joined by letter. The bearer of these presents, coming to the Church of the blessed Peter, Prince of the apostles, asserted that he had received letters for us from your Fraternity, and had lost them, with other things also, in the city of Jerusalem. In them, as he says, you were desirous of enquiring with regard to priests and people who have been bewildered in the error of Nestorian heresy, when they return to the Catholic Church which is the mother of all the elect, whether they should be baptized, or joined to the bowels of the same mother Church by confession only of the one true faith.
And indeed we have learned from the ancient institution of the Fathers that whosoever among heretics are baptized in the name of the Trinity, when they return to holy Church, may be recalled to the bosom of mother Church either by unction of chrism, or by imposition of hands, or by profession of the faith only. Hence the West reconciles Arians to the holy Catholic Church by imposition of hands, but the East by the unction of holy chrism. But Monophysites and others are received by a true confession only, because holy baptism, which they have received among heretics, then acquires in them the power of cleansing, when either the former receive the Holy Spirit by imposition of hands, or the latter are united to the bowels of the holy and universal Church by reason of their confession of the true faith. Those heretics, however, who are not baptized in the name of the Trinity, such as the Bonosiaci and the Cataphrygæ, because the former do not believe in Christ the Lord, and the latter with a perverse understanding believe a certain bad man, Montanus, to be the Holy Spirit, like whom are many others—these, when they come to holy Church, are baptized, because what they received while in their error, not being in the name of the Holy Trinity, was not baptism. Nor can this be called an iteration of baptism, which, as has been said, had not been given in the name of the Trinity. But the Nestorians, since they are baptized in the name of the Holy Trinity— though darkened by the error of their heresy in that, after the manner of Jewish unbelief, they believe not the Incarnation of the Only-begotten— when they come to the Holy Catholic Church, are to be taught, by firm holding and profession of the true faith, to believe in one and the same Son of God and man, our Lord God Jesus Christ, the same existing in Divinity before the ages, and the same made man in the end of the ages, because The Word was made flesh and dwelt among us Jn. 1:14. (Epistles, Bk. 11: Epistle 67)