20. The specification of "Israel," as well as Judah, shows the reference is to times yet to come.
iniquity … none—not merely idolatry, which ceased among the Jews ever since the Babylonian captivity, but chiefly their rejection of Messiah. As in a cancelled debt, it shall be as if it had never been; God, for Christ's sake, shall treat them as innocent (Jer 31:34). Without cleansing away of sin, remission of punishment would be neither to the honor of God nor to the highest interests of the elect.
whom I reserve—the elect "remnant" (Isa 1:9). The "residue" (Zec 14:2; 13:8, 9).