Women write prayer messages on ribbons outside the Cathedral Basilica of Saints Peter and Paul, as part of an art installation called "Knotted Grotto" in Philadelphia, Pennsylvania, September 23, 2015. The events expected to draw the biggest crowds during Pope Francis' six-day visit to the U.S. all occur in Philadelphia, September 26-27. REUTERS/Mark Makela 

Three nuns cross the street towards the Cathedral Basilica of Saints Peter and Paul in Philadelphia, Pennsylvania, September 23, 2015. The events expected to draw the biggest crowds during Pope Francis' six-day visit to the U.S. all occur in Philadelphia on September 26-27. REUTERS/Mark Makela