from the teachings of the Rebbe on the Torah portion
This Shabbat is known by two names:
Shabbat Shuva, derived from the opening words of the Haftorah that is read in synagogue, "Shuva Yisrael--Return, O Israel," and
Shabbat Teshuva, as it falls out in the middle of the Aseret Yemei Teshuva, the Ten Days of Repentance. This name is also connected to the Haftorah, the theme of which is likewise the return to G-d.
The two names of this Shabbat reveal a timely lesson.
The word "shuva -- return" is the command form of the word "lashuv -- to return."G-d commands us to return to Him in teshuva.
"Teshuva," by contrast, is a noun denoting the action itself, the actual return to G-d.
The name "shuva" relates more to the One who is issuing the command than the person being addressed.
"Shuva" alludes to a situation in which the command has already been issued, but not yet carried out. The command itself imparts a measure of strength but does not ensure that it will necessarily be fulfilled in the future.
The name "teshuva," on the other hand, implies that the action has already been taken, i.e., teshuva has already been done. In that case, however, why do we continue to refer to this Shabbat as Shabbat Teshuva?
The answer is that the act of teshuva consists of both the command to return to G-d and its subsequent implementation.
"Shuva" teaches us that even after a Jew has done teshuva, he still needs to work on himself to an even greater degree. No matter how much teshuva a person has done, it is always possible to rise higher; hence the directive, "Return, O Israel unto the L-rd, your G-d."
In fact, our teshuva must be "unto the L-rd, your G-d." Thus it is understood that there is always room for improvement -- for an even deeper and infinite teshuva -- as G-d Himself is Infinite.
This is the lesson of Shabbat Shuva: A Jew must never content himself with his previous Divine service and spiritual advancement. He must never think that because he has worked on himself a whole week he is now entitled to "rest" because it is Shabbat. No, today is "Shabbat Shuva!" Even after one has done teshuva, more work is required! For the service of teshuva is continual and without end.
Adapted from Hitva'aduyot 5744 of the Rebbe, volume 1