Rashi: Even in their Galus, my Torah will not be forgotten from them.

Radak citing Ibn Ezra: It is Ru'ach Nevu'ah that they will have, i.e. "Ruchi Asher Alecha."

Radak citing his father: It is the Torah, which was given with a Bris and a promise that it will never cease from their mouths.

Radak: Because they will be without sin - "ul'Shavei Fesha" (20), I promise them a new Bris - "v'Charati Es Beis Yisrael v'Es Beis Yehudah Bris Chadashah; ... v'Nasata Es Torasi b'Libam; ... Kulam Yed'u Osi lemi'Ketanam v'Ad Gedolam" (Yirmeyah 31:30, 32-33).

Refer to 59:21:1:2.

Radak: It is a spirit of Taharah that I will put in them, that they will not sin again, like it says "v'Es Ruchi Eten b'Kirbechem v'Asisi Es Asher b'Chukai Telechu" (Yechezkel 36:27). The Ru'ach that was in the Navi, I will put it in Yisrael.

Malbim: It is the Nevi'im's words.

Radak #1: The matter is repeated. This is like "Eshpoch Es Ruchi Al Kol Basar" (Yo'el 3:1).

Radak #2: The words that were in your [Yeshayah's] mouth, I will put them in Yisrael's mouth, and they will never cease from then and their seed.

Malbim: It is Divrei Torah, which Moshe put in front of Bnei Yisrael.

Bava Metzi'a 85a: If a Chacham's son and grandson are Chachamim, Torah will never cease from his seed.

Radak #1: Nevu'ah will not cease [from Yisrael].

Radak #2: Refer to 59:21:4:2.

Malbim: Initially, Yisrael had Nevu'ah, until Chagai, Zecharyah and Malachi. At the end of Nevu'ah, it says "Zichru Toras Moshe Avdi" (Mal'achi 3:22). The oral tradition of Torah is the continuation of Devar Hashem.

Bava Metzi'a 85a: Hashem made himself a guarantor for this.

Bava Metzi'a 85a: From this point (when a man, his son and grandson are all Chachamim), Torah returns to its host.