Continuity mistake: When the choir are singing the first song that Deloris helps them with, Sister Mary Robert gains confidence and sings louder. Toward the end of the song, she puts her hands together in a praying gesture while she is singing loudly, but in the next shot her arms are by her side.
Continuity mistake: The very first second we see old Deloris, during the opening credits, she holds the mic with her right hand, then lowers her left hand. A shot later, her left hand is now on the mic, and the right hand is grabbing the microphone stand.
Continuity mistake: When Willie and Joey are pointing their guns at Delores in Vince's office, they count to three, waiting for the other to shoot first. In the close-up shot, Willie's gun is closer to Delores' head than it was in the previous wide shot.
Continuity mistake: When Deloris is on TV, we see her lift a stuffed rabbit to try and hide her face, she then gives it to Mary Robert and lifts the kid to her face but when Eddie says "I'm gonna kill her. I'm gonna kill her myself", if you look on the TV, you can see her doing the exact same thing again.
Answer: The event wasn't a Mass, it was a concert that the Pope requested. So neither he nor anyone else is there to worship; they're just there to hear the nuns sing.
ksc