Question:

The Zohar writes that the teshuvah of even one Tzadik is enough to bring the Redemption.  Is it possible that over all the generations, there has not been even a single Tzadik who did complete teshuvah?

Answer:

The Zohar is referring to an exalted level of teshuvah, which no Tzadik has yet reached.

It is written in the book of Zohar, in a section called Zohar Chadash:  "If the heads of the communities, or even the head of one community, will do teshuvah, the exiles will be gathered in."  Rabbi Shneur Zalmen quotes this Zohar in a slightly different form:  "If only one Tzadik would do perfect teshuvah, Moshiach would come in his generation."

To understand the meaning of the Zohar, we must first define the term "teshuvah." Teshuvah does not simply mean repentance for sins and misdeeds.  It refers to restoring the soul to the lofty spiritual level it was on before descending to earth. 

When Adam and Eve sinned by eating from the Tree of Knowledge, good and evil became intermingled.  Since that time, in every good there is a slight amount of evil mixed in.  Even the spiritual worlds have subtle traces of evil, which will not be eradicated completely until Moshiach comes.

A Tzadik who does complete teshuvah is one who has succeeded in elevating his soul to the supernal heights, beyond even the slightest hint of evil.  The Geulah will begin with this accomplishment.  When one person succeeds in purifying himself of every last trace of evil, this shows that evil is in retreat, and the good is being sifted out of it -- which is the definition of Redemption.

Even a complete Tzadik, whose whole existence is his love and fear of Heaven, and has even reached the heights of G-dly revelation, still cannot be said to have done complete teshuvah, in the sense of the Zohar.  Once a Tzadik succeeds in reaching this level, the floodgates will be open and G-dly light will flow to the entire universe, and the Redemption will have arrived.