Thou shalt fan them - Keeping up the figure commenced in the previous verse. To fan here means to winnow, an operation which was performed by throwing the threshed grain up with a shovel into the air, so that the wind drove the chaff away. So all their enemies, and all the obstacles which were in their way should be scattered.
And the whirlwind shall scatter them - The ancients believed that people might be swept away by a storm or whirlwind. See Job 27:
The cast wind carrieth him away and he departeth;
And as a storm hurleth him out of his place.
Compare Homer, Odys. xx. 63ff, thus rendered by Pope:
Snatch me, ye whirlwinds! far from human race,
Tost through the void illimitable space;
Or if dismounted from the rapid cloud,
Me with his whelming wave let ocean shroud!
See the notes at Job 30:22.
And thou shalt rejoice in the Lord - In view of the aid which he has vouchsafed, and the deliverance which he has performed for you.
Shalt glory - Shalt boast, or shalt exult. You will regard God as the author of your deliverance, and joy in the proofs of his interposition, and of his gracious protection and care.