The current collegiate church arises from the superposition of different churches modified in the thirteenth, fifteenth and seventeenth centuries, using materials from the ancient cathedral, from which the Baroque style remains. It is dedicated to the hermit Pietro da Trevi. The organ of 1633 is by Ennio Bonifazi, a famous organ builder of the seventeenth century. It has a frontispiece with 3 bays, each with the central pipe twisted, powerful and sweet registers, which have retained the tonal freshness. There are carved and gilded wooden friezes,. The cost of the organ was partly paid for by the Community Trevi nel Lazio.