The core of the explanation of this book is the conflict between Catholicism and Protestantism and the conflict between the working class and the upper class. Vermeer was a renowned Dutch painter in the Dutch Golden Age. Besides Girl with a Pearl Earring, another famous Vermeer is The Milkmaid. Tracy Chevalier invented a backstory for the girl with the earring. Griet never existed but did represent the socioeconomic environment of the time (c. 1665) and did represent the human element living within the milieu.
Protestantism had shown itself to be vehemently, even violently, opposed to the Catholic Church when the iconoclasts (i.e., destroyers of established images) of the 1500s ravaged Catholic churches and destroyed every icon and statue they could touch. Vermeer's painting establishes the girl with the earring as Protestant through articles of clothing, like her artistically draped turban and the rough fabric of her humble dress. A distinguishing practice of Protestants was the refusal to pierce their ears because of the New Testament verse admonishing against external adornments:
Your beauty should not be an external one, consisting of braided hair or the wearing of gold ornaments .... (International Standard Version i Peter 3:3)
In contrast, Catholic women regularly pierced their ears. They also wore their hair exposed, while Protestant women wore their hair under a cover. The painting presents a startling contrast between the physical signs of lower class Protestantism and upper class Catholicism: a pearl earring was an elegant luxury as pearls had to be found in clams after they were harvested by tropical divers.
Chevalier weaves the painting's physical contrasts into her story that is precisely meant to explain these contrasts. So, if we have a working class Protestant girl working in a wealthy Catholic household, there must be latent hostility and fear and at least subtle rejection. When a Protestant girl is convinced by her Catholic employer (Vermeer converted to Catholicism to marry) to violate her religion and pierce her ear to wear his wife's costly earring, there is going to be tremendous emotional agitation and uncertainty underlying questions about loyalty, friendship, belief, and duty. These two conflicts--religious and socioeconomic--and the deep emotion that accompanies each, explain the story, particularly the part where Griet pierces her ear.
"Clove oil," he said at last with a sigh ... Rub a little on the spot ...."
... All I could really see [of the painting] now was the great hole where the earring should go, which I would have to fill. ...
... My eyes were full of liquid in the candlelight, glittering with fear.
Do this quickly, I thought. It will not help to delay.