Aquileia was the second city of the Western Empire, and by some called the second Rome. It was the Metropolis of Istria, Forum Julium, and Venetia; and its subjection to the See of Rome is manifest by the decretal Epistle of Leo I. directed to Nicetas Bishop of this city; for the Pope begins his Epistle thus: Regressus ad nos filius meus Adeodatus Diaconus Sedis nostrae, dilectionem tuam poposcisse memorat, ut de his a nobis authoritatem Apostolicae Sedis acciperes, quae quidem magnam difficultatem dijudicationis videntur afferre. Then he sets down an answer to the question proposed by Nicetas, and concludes thus: Hanc autem Epistolam nostram, quam ad consultationem tuae fraternitatis emisimus, ad omnes fratres & comprovinciales tuos Episcopos facies pervenire, ut in omnium observantia, data prosit authoritas. Data 12 Kal. Apr. Majorano Aug. Cos. A.C. 458. Gregory the great A.C. 591, cited Severus Bishop of Aquileia to appear before him in judgment in a Council at Rome.
The Bishops of Aquileia and Millain created one another, and therefore were of equal authority, and alike subject to the See of Rome. Pope Pelagius about the year 557, testified this in the following words: Mos antiquus fuit, saith he, ut quia pro longinquitate vel difficultate itineris, ab Apostolico illis onerosum fuerit ordinari, ipsi se invicem Mediolanensis & Aquileiensis ordinare Episcopos debuissent. These words imply that the ordination of these two Bishops belonged to the See of Rome. When Laurentius Bishop of Millain had excommunicated Magnus, one of his Presbyters, and was dead, Gregory the great absolved Magnus, and sent the Pallium to the new elected Bishop Constantius; whom the next year he reprehended of partiality in judging Fortunatus, and commanded him to send Fortunatus to Rome to be judged there: four years after he appointed the Bishops of Millain and Ravenna to hear the cause of one Maximus; and two years after, viz. A.C. 601, when Constantius was dead, and the people of Millain had elected Deusdedit his successor, and the Lombards had elected another, Gregory wrote to the Notary, Clergy, and People of Millain, that by the authority of his Letters Deusdedit should be ordained, and that he whom the Lombards had ordained was an unworthy successor of Ambrose: whence I gather, that the Church of Millain had continued in this state of subordination to the See of Rome ever since the days of Ambrose; for Ambrose himself acknowledged the authority of that See. Ecclesia Romana, saith he, hanc consuetudinem non habet, cujus typum in omnibus sequimur, & formam. And in his Commentary upon 1 Timothy 3:Cum totus mundus Dei sit, tamen domus ejus Ecclesia dicitur, cujus hodie rector est Damasus. In his Oration on the death of his brother Satyrus, he relates how his brother coming to a certain city of Sardinia, advocavit Episcopum loci, percontatusque est ex eo utrum cum Episcopis Catholicis hoc est cum Romana Ecclesia conveniret? And in conjunction with the Synod of Aquileia A.C. 381, in a synodical Epistle to the Emperor Gratian, he saith: Totius orbis Romani caput Romanam Ecclesiam, atque illam sacrosanctam Apostolorum fidem, ne turbari sineret, obsecranda fuit clementia vestra; inde enim in omnes venerandae communionis jura dimanant. The Churches therefore of Aquileia and Millain were subject to the See of Rome from the days of the Emperor Gratian. Auxentius the predecessor of Ambrose was not subject to the see of Rome, and consequently the subjection of the Church in Millain began in Ambrose.