It is important to remember that this work is actually a satire, and therefore, although it ostensibly draws our attention towards the subject matter of Persian harems through the two central characters of Usbek and Rica, its central theme is actually a very clever satire of French satire as these two foreigners journey into Europe and make various observations about French society from their perspective as outsiders. The theme of this text is therefore the moral decline and decadence that became firmly rooted in France from 1712 in the last years of the reign of Louis XIV and during the minority of Louis XV.
To see this satire in action we need look no further than the observations that Rica makes about the society he travels through. For example, he claims that the King of France was richer than the King of Spain. Even though the King of Spain had vast mines of gold and silver at his disposal, the French King was more wealthy because of the vanity of his subjects, which was a greater source of wealth. Rica argued that the gullibility of the French citizenry was so great that the French King had only to say that paper was the currency and the French people would be satisfied.
Therefore, Montesquieu uses the device of two foreigners travelling abroad to comment critically upon French society and its various excess. The clear theme is this satirical purpose, as French society and its various failings are mercilessly exposed and ridiculed.