Question: I’ve learned that only Joseph and Noah are called Tzaddikim – the term for “righteous.”, in the entire Torah. What is the similarity in the way they completed their missions for Hashem that allows them both to have this title?
Answer: As Joseph is actually not referred to as “Tzadik” in either the Bible or the writing of the Sages, it is rather difficult to be confident that I can fully understand the origin of this particular custom.
I would suggest, however, that you might think about how Rabbi S.R. Hirsch explains that the term “Tzedek” involves ensuring that everyone and everything is given whatever it deserves. That means that a Tzadik will carefully think about all his abilities, resources and talents and faithfully use them all to precisely fulfill G-d’s will. If someone manages to do this his entire life; to constantly struggle for G-d’s interests despite opposition, then he will have become a Tzadik.
I hope that this is useful.
With best wishes,
Rabbi Boruch Clinton