What is the spiritual meaning of Nathan the Wise?

Lessing's overriding message in Nathan the Wise is the importance of religious toleration. As a leading figure of the Enlightenment, Lessing fervently believed that religious divisions had caused so much suffering in the world that a new spirit of mutual tolerance was necessary if the various faiths were to live together in harmony.
The play is set in Jerusalem during the late 12th century CE, around the time of the Third Crusade. Then, as now, the Holy City was a hotbed of inter-religious tensions. Muslims, Jews, and Christians alike all venerated Jerusalem as a sacred place. This led to regular outbreaks of conflict between adherents of these three monotheistic creeds, greatly undermining the city's peace and stability.
Saladin, the ruler of Jerusalem in Nathan the Wise, is a Muslim, and like everyone else, he thinks his religion is the only true one. But in his capacity as sultan, he also knows that it's in his best interests to maintain good order in the city. To that end, he maintains a close friendship with Nathan, a Jewish merchant. During one of their conversations, Saladin demands that Nathan tell him which is the "true" religion. In response, Nathan tells Saladin a fable about a man with three sons whom he loves equally. Unable to decide which of his children should receive his valuable gold ring, he has two copies made. But his sons are dissatisfied; each wants to know for sure if he has the original ring or a copy. They appeal to a judge to settle the matter. In his wise verdict, the judge rules that each son should act as if the original ring were his own and bring out the ring's magical power—the ring confers love of both God and man—by performing good deeds.
The spiritual message of Nathan's ring fable is that it's not so much which religion you follow, but how you act toward others, that matters. Everyone should remain loyal to their chosen faith while at the same time striving to act morally at all times. In this way, the differences between the religions will be minimized, and people of different faith traditions will be able to live together more harmoniously.