Another minor note. Not condoning or justifying slavery in any way, but if it were not for slavery, most African Americans would not EXIST. The very people who complain about past sins ("America's Original Sin") would not be alive in some desolate village in West Africa; they would not exist at all! The precise combinations of ancestors which led to their existence would not have happened. All in all, whether they like it or not, they owe their very existence to the despicable practice of slavery. They are not alone in this, of course. All of us have in our genealogies, crimes, some caused in war, some not, that for better or ill, led to our being here.
Actually, the Pope, as the head of the independent Papal States, abolished slavery in 1533. He called for all Christian nations to abolish it after the reports of Spanish slavery in the New World.