I don't believe in the original sin of slavery in Western countries.
Slavery has been existed for centuries before and, despite being abolished by Western countries (hint: there were several wars were people lost lives), still exists in China, India, Indonesia, and a large group of Arab countries.
> I don't believe in the original sin of slavery in Western countries.
I have no idea what that sentence is specifically meant to mean, that aside, western countries have had actual slavery and effective slavery both in the past and the present.
I don't mean to exclusively beat on Christians or just Catholics here; but I do intend to counter your apparent narrative of good Christians and evil Muslims.
Christians certainly diminished a lot of cultures, just as other religions also did - the initial point that started this above.