TREVI NEL 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 of its diocesan life date back to the 5th-6th centuries.
It is very difficult to draw up the list of its bishops who took part in the Roman Councils of the Middle Ages, because they are often confused with those of the city of the same name in Umbria, situated on the Via Flaminia, itself also a very 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 Treba extended from east to west, starting from Filettino as far as 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. Joined to the Cathedral was the episcopal Palace where, in 1260, Cardinal Hugh of St. Sabina spent a good part of the summer and autumn and here received as a guest Cardinal Giacomo Pantaleone of Troyes, who in 1261 became pontiff under the name of Urban IV. T., as a diocese, led a struggling existence owing to the poverty of the territory over which it extended its jurisdiction. In the 11th century these conditions worsened because of the failure of agriculture and commerce to revive, which at this time animated other regions of Italy. Nicholas II in the years 1059-61 suppressed the diocese of T.: the bull of Urban II of 1088 has come down to us, which refers 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 to be united to Subiaco. The opposition of the people of Treba to the suppression of their diocese was energetic, constant, continuous; almost all the popes, from 1061 to 1227, that is for about 166 years, dealt with this dispute. Gregory IX, in 1227, definitively confirmed the union of T. and the dependent towns with Anagni, granting the abbot of S. Teodoro certain privileges.