Do you think if the catholic church took a strongly anti-slavery stance in the 1500's they could have stopped slavery at least on the catholic territories of the americas? What would be the major consequences of that later?
Then they'd replace it with something that is slavery in all but name, which was in fact what the Spanish did with their encomienda system or the forced labour the Jesuits used in their missions. It could probably even work the same way, where the not!slave traders go to Africa and buy the freedom of slaves on the condition they become indentured servants to pay off their debts. I can imagine the vague prospect of freedom sounds more appealing than a life of slavery, so they'd find plenty of volunteers. And to the African powers engaged in the slave trade, they'd act just as OTL to ensure there was always a large number of slaves ready for sale.
Reason being is that slavery was too highly demanded and too profitable in brutal operations like tropical sugar production and mining once the native population collapsed in part due to the brutality of said mining and plantation agriculture. If the Catholic Church bans slavery, then it will be replaced with something else lest Protestant states like England or the Netherlands gain an advantage.
The one interesting factor is that even if slaves are coerced into signing contracts for lifetime indenture, it would be harder to coerce their children into that same system. That would have a huge effect on the African diaspora in the New World where there'd be way more free blacks. And would the Spanish/Portuguese/French/whoever be importing as many women? Would there be more mixing between black and indigenous populations?
Was there ever a possibility of a Pope with legitimate children? I.e. a man who marries young, has children, loses his wife, enters the Church, and rises to be Pope? St. Francis Borgia followed such a path, and became Superior General of the Society of Jesus (the head Jesuit). Bonus points if the Pope's child or grandchild is a king.
Peter has surviving children? The Bible clearly says he was married, but there's no mention of him having children or any tradition of his children so they either died young or he/his wife was infertile.