About my diocese (Trevi nel Lazio)
A little known fact about auxiliary bishops is that they have a diocese of their own too. In my case, I am an auxiliary bishop for Montreal but also the titular bishop of Treba in Italy (today known as Trevi nel Lazio, in the province of Frosinone).
You may be wondering where this notion of a "titular" bishop come from. In every case a titular diocese was once a regular diocese with its own diocesan bishop. However, in most cases some war or other calamity either required the bishop to flee, or required the pastoral care of those people pass to someone else. As well, because travelling was often difficult some parts of the world had a great many small dioceses, which eventually got merged. This last scenario likely happened to my diocese at some point. Now that the jurisdiction of this territory has passed to another local Italian diocesan bishop, the title can be used to appoint an auxiliary bishop elsewhere in the world…such as myself, in Montreal.
I should point out that not everyone believes in the use of titles like this for auxiliary bishops — after all, could we not just appoint priests as auxiliary bishops without this sort of legal fiction? I do think that there is perhaps room for development in theology and canon law in this area, especially as we continue to explore the concept of a “local Church” in our ecumenical dialogue with the Orthodox.
All this being said, though, while I have no living members of the diocese to take care of, I do plan on saying mass on November 2 (the feast of All Souls) for the deceased members of that local Church. The Treba diocese did exist for 500+ years (longer than Canada!) and who knows? Maybe there is still a soul or two waiting for someone to pray for them and help them out of purgatory. Who better to offer that prayer than the bishop of that historical place?