Why is a new Bris needed?
Radak: The first Bris, which Hashem made with Yisrael at Har Sinai, was annulled 1 (verse 31). There will be a new Torah 2 , due to their Teshuvah. The new Bris will not be annulled, for I will put it in their hearts and write it on their hearts, lest it be forgotten. The only Chidush of the Bris is its fulfillment. Mal'achi, the last Navi, closed his Nevu'ah with "Zichru Toras Moshe Avdi Asher Tzivisi Oso v'Chorev Al Kol Yisrael Chukim u'Mishpatim" (Mal'achi 3:22). That is written for the future, like it says "Hine Anochi Shole'ach Lachem Es Eliyahu ha'Navi" (3:23). This shows that it will not be a new Torah, rather, the same "Asher Tzivisi Oso v'Chorev."
Malbim: The previous Bris was on condition that they not sin. Sin annuls the Bris. ("No person does good, and does not sin" (Koheles 7:20)! Perhaps only idolatry annuls the Bris, or if most of the nation have more sins than merits. - PF)
The Rambam's ninth principle of Emunah is that Hashem will not change the Torah! Radak retracts from this below. Perhaps his Havah Amina was that the only reason to require a new Bris is if there will be a new Torah. (PF)
