Aeterna Dei Sapientia §63

⁶³ Our fervent appeal for unity is intended, therefore, to be the echo of that which was made many times by St. Leo in the fifth century. We wish, too, to make Our own those words which St. Irenaeus addressed to the faithful of all the churches, when God's Providence called him from Asia to rule the See of Lyons and confer on it the fame of his martyrdom. Recognizing that the Bishops of Rome were heirs to that power which had been handed down in uninterrupted succession from the two Princes of the Apostles,(57) he went on to address the following appeal to all Christians:—