Origen

Or, because without the wind the wheat and chaff cannot be separated, therefore He has the fan in His hand, which shews some to be chaff, some wheat; for when you were as the light chaff; (i. e. unbelieving,) temptation shewed you to be what you knew not; but when you shall bravely endure temptation, the temptation will not make you faithful and enduring, but it will bring to light the virtue which was hid in you.