Joel 3:21

21. For I will cleanse their blood that I have not cleansed The meaning is expressed more clearly in margin of R.V., “And I will hold (or, declare) as innocent their blood, that I have not declared innocent.” For [“And”]-Better, And so, that is, by the execution of judgment upon Edom and Egypt and the showering of abundant blessings upon Judah and Jerusalem. “I will declare as innocent” The Hebrew is chosen with reference to the expression “innocent blood” in Joel 3:19 (compare Exodus 20:7; Job 9:28). So long as Jehovah permitted the Jews to suffer and their enemies to prosper it might be supposed that the former had deserved all their afflictions; but the judgment sent as punishment upon the oppressors is decisive proof of the latter’s guilt, and so implies the innocence of the Jewish victims. The emendation of Nowack, “and I will avenge their blood, nor will I suffer (the enemies) to go unpunished,” is unnecessary. Jehovah dwelleth in Zion (Joel 2:27; Joel 3:17) This clause does not state the reason why all the promises will be fulfilled namely, because Jehovah is in Zion; nor is it a corroboration of the preceding promise: this shall come to pass as truly as Zion is the habitation of Jehovah; rather a reiteration of the greatest of all promises: The judgment executed, Jehovah will establish himself in Zion forever; never again will he forsake his people so that they become a reproach among their enemies.

The prophecy of Joel opens with a picture of utter hopelessness and despair; it closes with the promise that even the highest hopes of the most optimistic Jews shall be realized in all their fullness.