The church lies in the place where the theatre of the ancient Roman city was located. The original Christian sacred house included three churches, dedicated to the Holy Saviour, Saint Mary of Dompno () and, the main of three, St. John the Baptist. According to some sources, the latter's consecration was carried on by Agilulf, the Lombard King of northern Italy from 591 to 613. Here, in 662, Garibald, Duke of Turin (it) was assassinated in the church by a follower of Godepert, whose murder Garibald is believed to have had a part in.