By this — By this manner of God's dealing with them. When — Which sin of Jacob's shall be purged, when he shall truly repent of all his sins, and especially of his idolatry. Altar — Their idolatrous altars. Possibly he may say the altar, with respect to that particular altar, which Ahaz had set upon the place of God's own altar; and this prophecy might be delivered in Ahaz's time, while that altar stood. Chalk — stones — When he shall break all those goodly altars in pieces. Not stand — Shall be thrown down with contempt.
Related Commentaries of Isaiah 27:9
Isaiah 27:9
9 By this therefore shall the iniquity of Jacob be purged; and this is all the fruit to take away his sin; when he maketh all the stones of the altar as chalkstones that are beaten in sunder, the groves and imagesc shall not stand up.