According to the Christian tradition the church was built in the place where Jesus appeared to Pietro, who just escaped from the city because he didn't want to be crucified.When the apostle asked "domine quo Vadis?" (Where are you going to my Lord?), the Christ answered" Venio iterum crucifigi" (i'm coming back in Rome so as to be crucified). That's why Pietro accepted to become a martyr Hence the fame of this church.