I have however taken care to admonish earnestly the same my brother and fellow-bishop that, if he desires to have peace and concord with all, he must refrain from the appellation of a foolish title. As to this, the piety of my lords has charged me in their orders, saying that offence ought not to be engendered among us for the appellation of a frivolous name. But I beseech your imperial Piety to consider that some frivolous things are very harmless, and others exceedingly harmful. Is it not the case that, when Antichrist comes and calls himself God, it will be very frivolous, and yet exceedingly pernicious? If we regard the quantity of the language used, there are but a few syllables; but if the weight of the wrong, there is universal disaster. Now I confidently say that whosoever calls himself, or desires to be called, Universal Priest, is in his elation the precursor of Antichrist, because he proudly puts himself above all others. Nor is it by dissimilar pride that he is led into error; for, as that perverse one wishes to appear as above all men, so whosoever this one is who covets being called sole priest, he extols himself above all other priests. But, since the Truth says, “Every one that exalteth himself shall be humbled,” (Luke 14:11; 18:14), I know that every kind of elation is the sooner burst as it is the more inflated. Let then your Piety charge those who have fallen into an example of pride not to generate any offence by the appellation of a frivolous name. For I, a sinner, who by the help of God retain humility, need not to be admonished to humility. Now may Almighty God long guard the life of our most serene Lord for the peace of holy Church and the advantage of the Roman republic. For we are sure, that if you live who fear the Lord of heaven, you will allow no proud doings to prevail against the truth.
Gregory I, Epistle XXXIII. To Mauricius Augustus.
Can be found here