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In Europe during the Medieval times the only recognised religion was Christianity, in the form of the Catholic religion. The lives of the Medieval people of the Middle Ages was dominated by the church. From birth to death, whether a peasant, a serf, a noble a lord or a King - life was dominated by the church and Medieval religion."
Medieval Religion
It was pretty hard if not impossible to find a European person who wasn't Christian back then. Meanwhile, it was very easy to find African people to enslave. To me, it's clear that European leaders didn't want their people to be sold to outsiders. They didn't mind if they were sold to other Europeans.
There was nothing in Africa that unified most black people. I understand some African leaders took measures to stop slavery, but 20 million people got sold to outsiders with disastrous effects.
Medieval period lasted to the XVth century. That's not the period we're talking about.
And the full quote of the link you provided says:
Disputes of the Crusades led to the split between the Eastern and Western Christian Churches, called the Great Schism of 1054. The practises of the Catholic religion were questioned and the beliefs of men such as Martin Luther (1483 - 1546) prompted a new religion called Protestantism which led to a further split in the Christian Church referred to as the Protestant Reformation.
which is part of what I referred to when I said Europe was not united in religion. These splits led to numerous wars and conflicts amongst Europeans and atrocities committed against each other.
That "in the form of Catholic religion" is important, as well as "recognized", because that doesn't mean that other cults and beliefs didn't exist, but that they were forbidden by law.
It was far from impossible to find non-Christian Europeans:
History of European Jews in the Middle Ages - Wikipedia
Islam in Europe - Wikipedia
And again, the question is not even about Christian Europeans, but the subdivisions that exist in Christianity and allowed Christian Europeans to fight, kill, exile etc. each other.
I don't see your point in claiming that Europeans didn't care to sell each other between each other, but not to non-Europeans. Besides the Moors, non-Europeans didn't care much for European slaves to the point a large-scale trade would have worked. The quasi-absence of demand doesn't imply that Europeans would have been opposed to it. History shows the contrary given that they committed all the atrocities possible to each others.
Idk where you're going with this.