Commentaries:
Twice in this four-verse section God says that, when the Israelites return to the Promised Land in the Millennium and finally begin to repent, then they will be able to put things together properly. Similar phraseology is also used in the prophecies of Joel, Zechariah, and Malachi. Sometimes it is "then you shall know that I am the Lord." Other times, it is "then you shall know that a prophet has been among you" or "then you shall know that the Lord has sent me." But it is always implies understanding after the fact, that is, after a prophecy has occurred. Only then do we really come to understand what exactly took place.
Richard T. Ritenbaugh
The Two Witnesses (Part One)