An agnostic asked, what is one Christian standard that differentiated European kings of the Middle Ages from Muslim/Chinese kings?

My answer: Because of Christianity, each European king of the Middle Ages had only one wife, while all contemporary Muslim kings had many wives. The Chinese Emperor also had many wives and concubines, as witnessed by Marco Polo.

What is remarkable is that this Christian standard over time has influenced the whole world, to the point that nowadays, only Muslims kings have multiple wives, in places like Saudi Arabia.