This is the Abbey Church and was consecrated in 1024 by Pope John XIX of Tusculum. The church is brick with the classic Romanesque style with a bell tower and a portico with travertine columns. The facade has a rose window of the thirteenth century. Inside it is divided into three naves and at the entrance there is the baptismal font, and a mosaic of the Madonna and St. John the Baptist. The church has been renovated several times over the centuries and inside are frescoes of San Nilo and San Bartolomeo and paintings, only the apse mosaics belonging to the original construction. In the sixteenth century, with the Farnese, a coffered ceiling was built and the chapel renewed. In 1665, Cardinal Barberini built an "apparatus" to adorn the main altar with a thirteenth-century Madonna icon.