https://www.facebook.com/groups/archeologyandcivilizations/posts/2168641139896065/

> The recruits were indoctrinated into Islam, forced into circumcision and supervised 24 hours a day by eunuchs. They were subjected to severe discipline, being prohibited from growing a beard, taking up a skill other than soldiering, and marrying.

As a result, the Janissaries were extremely well-disciplined troops, and became members of the askeri class, the first-class citizens or military class.

Most were non-Muslims, because it was not permissible to enslave a Muslim.

Probably it was not permissible to enslave a jew either