TREVI in LAZIO. — Ancient diocese, near the sources of the Aniene, today in the diocese of Subiaco. It was a Roman municipium under the name of Treba Augusta. It was also an episcopal see, and the oldest documents concerning its diocesan life date from the 5th–6th centuries.
It is extremely difficult to compile a list of its bishops who took part in the medieval Roman Councils, because they are often confused with those of the town of the same name in Umbria, situated on the Via Flaminia and likewise an ancient diocese. In the 10th and 11th centuries, the first documents appear concerning relations between this diocese and the monks of Subiaco. The territory of the diocese of Trevi extended from east to west, from Filettino to Subiaco, following the course of the Aniene. The towns of Filettino, Vallepietra, Ienne, and Collalto belonged to its jurisdiction. The cathedral of T. was dedicated to the martyr St. Theodore and stood near the right bank of the Aniene, at the point where the Suria stream flows into this river, near the present Ponte S. Teodoro. Attached to the Cathedral was the episcopal palace, where, in 1260, Cardinal Ugo di S. Sabina spent much of the summer and autumn; here he received as a guest Cardinal Giacomo Pantaleone di Troyes, who in 1261 became pope under the name of Urban IV. T., as a diocese, led a precarious existence owing to the poverty of the territory over which it exercised jurisdiction. In the 11th century these conditions worsened because agriculture and commerce failed to revive, as they did in other regions of Italy during this period. Nicholas II suppressed the diocese of T. in the years 1059–61; the bull of Urban II from 1088 has survived, referring back to the decree of Nicholas II. T. and the four dependent towns were united to the diocese of Anagni, from which T. and Ienne were removed in 1638 in order to be united to Subiaco. The opposition of the people of Trevi to the suppression of their diocese was vigorous, steadfast, and persistent; almost all the popes, from 1061 to 1227—that is, for approximately 166 years—dealt with this dispute. In 1227, Gregory IX definitively confirmed the union of T. and the dependent towns with Anagni, granting the abbot of S. Teodoro certain privileges.