After entering the Middle Ages, Christianity had an enormous influence on marriage in the Western World. As the world's largest religion, Christianity's concepts spread continually with the increasing number of missionaries.
In Christian doctrine, the relationship between husband and wife is expected to be monogamous. Christianity deems marriage as the most important thing in life, a relationship established by God, sacred and pure, and should not be defiled.
At the same time, Christianity included marriage among the sacraments, as one of the seven sacraments. Thus, in the Western ancient civilizations, people's views on marriage were greatly influenced by Christianity.
However, this does not mean Westerners actually adhered to this principle. Historically, many kings and nobles had mistresses and illegitimate children. For instance, the most famous "Emperor John" of Portugal was an illegitimate child of his father.
