Charles Ellicott Commentary Isaiah 27:9

"Therefore by this shall the iniquity of Jacob be forgiven, and this is all the fruit of taking away his sin: that he maketh all the stones of the altar as chalkstones that are beaten in sunder, [so that] the Asherim and the sun-images shall rise no more." — Isaiah 27:9 (ASV)

By this therefore shall the iniquity of Jacob be purged. —The pronoun may refer either to the chastisement of the previous verse as the instrument of purification (preferably), or to the destruction of idols which follows as the result and proof of that purification, the end contemplated by Jehovah in His chastisements.

This is all the fruit to take away his sin. —Better, of taking away his sin. The words repeat the thought of the previous clause. The fruit of repentance and forgiveness will be found in rooting out all vestiges of idol-worship. The Septuagint, “when I shall take away their sins,” is quoted by St. Paul in Romans 11:27.

The groves and images. —Literally, as elsewhere, the Asherahs, or the sun-images, the two leading features of the cultus which Israel had borrowed from the Phoenicians. In the action of Josiah (2 Chronicles 34:3–4) we may, with little doubt, trace a conscious endeavor to fulfill the condition which Isaiah had thus proclaimed. He sought to “purge” Judah and Jerusalem from the “groves and the carved (sun) images, and molten images.”