Question:

How do we know that all Jews will be redeemed in the final Redemption?

Answer:

Rabbi Schneur Zalmen of Liadi, the founder of the Chabad Chassidic movement, rules: “Each and every Jew will certainly do Teshuvah in the end, in one incarnation or another, since no Jew will be lost.” What is the meaning of this promise that every Jew will eventually do Teshuvah?

One aspect of this promise is that when G-d gave the Torah on Mount Sinai, He chose each and every Jew to be part of His nation. G-d made a connection between Himself and the Jewish People, and as explained in the teachings of Chassidism, this connection is even stronger than the natural bond between a father and son.

Another aspect is that the Jewish People also bonded to G-d at Mount Sinai. The Jews chose to attach themselves to G-d as His servants. On the first verse of the Ten Commandments, “I am G-d your G-d Who took you out of the Land of Egypt,” Rashi comments: “By taking you out of Egypt, this is sufficient
for you to become subjugated to me.” At Mount Sinai, not only did G-d choose the Jewish people, but they also chose G-d, and committed themselves to follow His will and remain loyal to Him despite hardships and challenges.

All of us were partners to this agreement, as the Midrash relates that the souls of the entire Jewish nation, including all those who would ever be born, were present at Mount Sinai. Therefore, every Jew, without exception, “wants to be part of the Jewish people, and wants to fulfill all the mitzvot and avoid all the aveirot.” A Jew who acts differently has only temporarily fallen prey to his yetzer hara, his evil inclination, but this does not represent his true will or desire.

Therefore, even in the event that a Jew denies his relationship with G-d, he is unable to escape from it. Sooner or later, “in one incarnation or another,” it will reveal itself. The Jew will return in complete Teshuvah, and will thus merit the Redemption together with the entire Jewish people.

(Hilchos Talmud Torah, 4:3. Tanya, chapter 39. Likutei Sichos vol. 11, p. 1)