Radak: It is the future, like it says above (31:33) "Eslach la'Avonam ul'Chatasam Lo Ezkar Od", and "Kulam Yed'u Osi lemi'Ketanam v'Ad Gedolam." [It cannot be the return from Bavel, for] there were sinners in Bayis Sheni, and also now in our exile! Also, "Eslach la'Asher Ash'ir" is in the future, like it says "v'Hayah ha'Nish'ar b'Tziyon veha'Nosar bi'Yrushalayim Kadosh Ye'amer Lo" (Yeshayah 4:3), "She'eris Yisrael Lo Ya'asu Avlah" (Tzefanyah 3:13).

Malbim (refer to 50:20:2:1) implies that it is at the beginning of Bayis Sheni, for that is when the Yetzer ha'Ra for idolatry became Batel (Sanhedrin 64a). (PF)

Malbim: "Yisrael" is the 10 tribes. Idolatry is called Avon, for it demonstrates Ivus (crookedness) of intellect and denial. It will not be found, for the Yetzer ha'Ra for idolatry will be Batel.

Malbim: Yehudah did not deny Hashem. Their sin was due to lust, which is called Chatas. Even though it will be in the world, it will not be found, for Hashem will pardon the remnant that He leaves.