Epistle XVI. To Severus, Bishop of Aquileia 1317 . Gregory to Severus, &c. As, when one who walks through devious ways takes anew the right path, the Lord embraces him with all eagerness, so afterwards, when one deserts the way of truth, He is more saddened with grief for him than He rejoiced over him with joy when he turned from error; since it is a less degree of sin not to know the truth than not to abide in it when known:  and what is committed in error is one thing, but what is perpetrated knowingly is another.  And we, from having formerly rejoiced in thy being incorporated in the unity of the Church, are now the more abundantly distressed for thy dissociation from the catholic society.  Accordingly we desire thee, at the instance of the bearer of these presents, according to the command of the most Christian and most serene Emperor, to come with thy adherents to the threshold of the blessed Apostle Peter, that, a synod being assembled by the will of God , judgment may be passed concerning the doubt that is entertained among you. 1317     The bishops of Istria, of whom the bishop Aquilea was Metropolitan, still refused to accept the decree of the fifth Œcumenical Council, which had, under the dictation of the Emperor Justinian, condemned certain writings of three deceased prelates, Theodore of Mopsuesta, Theodoret and Ibas, called “the three chapters” ( tria capitula ).  Severus the Metropolitan, summoned in this letter with his suffragans to Rome, disregarded the summons, going instead, at the instance of the Exarch Smaragdus, to Ravenna, where he remained a year.  On his return to his See he still held out, though many of his bishops conformed.  A schism hence ensued in Istria, which continued during the life of Gregory (Joan. Diac. Vit. S. Greg. iv. 37, 38).  Other Epistles referring to the Istrian schism are II. 46, 51; V. 51; IX. 9, 10; XIII. 33.