Basilica San Paolo Fuori le Mure, or Saint Paul Outside the Walls, is one of Rome's largest and most important churches. As its name implies, the Basilica is outside Rome's walls so out of the tourist center but it's well worth visiting.
Highlights of the Basilica include a stunning 13th century mosaic, a carved marble Romanesque Easter Candle, and relics related to Saint Paul including his tomb and what are believed to be chains that were used on him during his arrest in Rome. There's also a beautiful cloister to visit, the Chapel of Relics, a small museum, and archeological finds.
Museo della Via Ostinese to the Basilica of Saint Paul is an interesting itinerary that starts at Porta San Paolo and goes to the church.
Saint Paul Basilica photo © by Martha Bakerjian, licensed to About.com