Well in the context of the time, slavery was common back then and was practiced under the Romans too. And as I said, converting legitimizes any previous illegitimacy as the Church was the successor to the Roman Empire.
So, enslaving and conquering Christians is fine (even for the sake of human sacrifice), except when Muslims do it. Furthermore, the Church is not the successor to the Roman Empire, it's a remnant of it. It was the religious branch of the Western Roman government, and nowadays is all that's left of it. Furthermore, if the approval of the Church legitimises any ruler, then not only was William of Normandy's claim the gold standard of legitimacy, but every usurper up to King Henry VIII was just as legit. Or does Church approval only legitimise when it's a former pagan conqueror?
I have to say, for a Christian supremacist, you seem to grant a lot of exclusive privileges to non-Christians.