“in Darwin’s day Europeans were fairly familiar with the Koran and the Haditsh (sayings of the Prophet).”
I find that surprising.
If you start conquering people you can make much more money off the deal if you try to understand their motivators.
Although your peasants in the field might not know anything about the Koran, the leadership elites in the European countries found it to their profit to know and did so.
Darwin lived in a world where he had regular contact with those elites.