Obviously the non-catholic denominations.
What did past Popes think of the Mormons, Jehovah's, and all other evangelical faiths when these movements started? In the eyes of the Catholic church, are/were they considered heretics?
Interesting fact: There have been 11 Popes since the founding of the United States
This list should help identify which Popes would be most likely to comment on historical events
If you don't get answers here (I know a couple of the flaired users are knowledgable about Papal encyclicals) you could try asking at /r/Catholicism.
Might be wrong, but maybe you could ask at r/Christianity