[nomadking]

A few topics for them to dwell on.

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Slavery in India escalated during the Muslim domination of northern India after the 11th-century, after Muslim rulers re-introduced slavery to the Indian subcontinent. It became a predominant social institution with the enslavement of Hindus, along with the use of slaves in armies for conquest, a long-standing practice within Muslim kingdoms at the time. According to Muslim historians of the Delhi Sultanate and the Mughal Empire era, after the invasions of Hindu kingdoms,

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India's caste system is among the world's oldest forms of surviving social stratification. The BBC explains its complexities.
The system which divides Hindus into rigid hierarchical groups based on their karma (work) and dharma (the Hindi word for religion, but here it means duty) is generally accepted to be more than 3,000 years old.
...
Rural communities have long been arranged on the basis of castes - the upper and lower castes almost always lived in segregated colonies, the water wells were not shared, Brahmins would not accept food or drink from the Shudras, and one could marry only within one's caste.

It was colonization of Africa that largely brought slavery to an end there.
The Black African rulers were only too happy to keep slavery.
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In 1807, Britain declared all slave trading illegal. The king of Bonny (in what is now the Nigerian delta) was dismayed at the conclusion of the practice.
"We think this trade must go on. That is the verdict of our oracle and the priests. They say that your country, however great, can never stop a trade ordained by God himself."

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It's thought that as many people were enslaved in the Eastern slave trade as in the Atlantic slave trade.
It's ironic that when the Atlantic slave trade was abolished the Eastern trade expanded, suggesting that for some Africans the abolition of the Atlantic trade didn't lead to freedom, but merely changed their slave destination.

Who is going to tell those in the Nation of Islam?

Muslim slavery continued for centuries
The legality of slavery in Islam, together with the example of the Prophet Muhammad, who himself bought, sold, captured, and owned slaves, may explain why slavery persisted until the 19th century in many places (and later still in some countries). The impetus for the abolition of slavery came largely from colonial powers, although some Muslim thinkers argued strongly for abolition.

Ottoman Empire

In the 14th century, the devshirme system was created. This required conquered Christians to give up 20 percent of their male children to the state. The children were forced to convert to Islam and become slaves.