How does the ring given to Schindler relate to the Holocaust and world war 2 in general?

Whoever saves one life saves the world entire.

Schindler's Jewish workers present a gold ring, on which the above Talmud quote is engraved, to Schindler before he leaves to flee from the Allies. The ring is made from a gold crown taken from one of the worker's teeth. The ring serves two functions. First, it expresses the appreciation and gratitude of Schindler's Jewish workers. They know that he will be hunted as a war criminal, and want him to know that they think of him as anything but that. Secondly, the quote on this ring puts forth the moral of the story. Spielberg wishes to convey this phrase as the message of the film.