St Peter's Basilica, Rome - Mass in St Peter’s Basilica is in Italian and makes for an amazing experience for anyone. It follows in a Catholic tradition going back centuries. http://www.8thingstodo.com/st-peters-basilica-rome
12th-century church of San Clemente (12; Via San Giovanni in Laterano; basilicasanclemente.com) has a lovely fresco by Masolino da Panicale (15th century). Steps lead down to the remains of a 4th-century basilica and, deeper, to a temple to the god Mithras.