Ambrose of Milan
Catena Aurea · Lukasevangelium 3
And as the people were in expectation, and all men mused in their hearts of John, whether he were the Christ, or not; John answered, saying unto them all, I indeed baptize you with water; but one mightier than I cometh, the latchet of whose shoes I am not worthy to unloose: he shall baptize you with the Holy Ghost and with fire. Whose fan is in his hand, and he will throughly purge his floor, and will gather the wheat into his garner; but the chaff he will burn with fire unquenchable.
By the sign of a fan then the Lord is declared to possess the power of discerning merits, since when the corn is winnowed in the threshing floor, the full cars are separated from the empty by the trial of the wind blowing them. Hence it follows, And he shall gather the wheat into his barn. By this comparison, the Lord shews that on the day of judgment He will discern the solid merits and fruits of virtue from the unfruitful lightness of empty boasting and vain deeds, about to place the men of more perfect righteousness in His heavenly mansion. For that is indeed the more perfect fruit which was thought worthy to be like to Him who fell as a grain of wheat, that He might bring forth fruit in abundance. (John 12:24.)