Albert Barnes Commentary Jeremiah 50:20

"In those days, and in that time, saith Jehovah, the iniquity of Israel shall be sought for, and there shall be none; and the sins of Judah, and they shall not be found: for I will pardon them whom I leave as a remnant." — Jeremiah 50:20 (ASV)

Those days – The days of the Messiah.

Reserve – Or, permit to remain: hence, the remnant, a word pregnant with meaning in the language of the prophets. See Isaiah 8:18 note (2).