Okay, so technically it's music that 17th-century Pope Urban VIII didn't want you to hear. Gregorio Allegri's Miserere was Urban VIII's jam—one of the most beautiful pieces of choral music ever composed. The pope loved the piece so much, in fact, that he banned it from being performed anywhere but the Sistine Chapel during Holy Week, punishable by excommunication.
The story goes that a young Mozart heard the piece in Rome in the late 18th century, and was so moved that he returned home and transcribed the entire piece from memory.
The Latin text comes from Psalm 51:
Have mercy on me, O God,
according to your steadfast love;
according to your abundant mercy
blot out my transgressions.
Wash me thoroughly from my iniquity,
and cleanse me from my sin.