Complex sentence: He pointed out that the main source of slaves was war, as prisoners of war were usually enslaved; when the Quran banned turning such prisoners into slaves, it effectively closed the door on the future of slavery.
Split into simpler sentences: He pointed out that the main source of slaves was war, as prisoners of war were usually enslaved. So when the Quran banned the enslavement of prisoners of war, it effectively closed the door on the future of slavery.