Oecumenius

And the angel said to him, Send forth your sickle, and reap: for the time to reap is come; for the harvest of the earth is ripe. And he says he cast his sickle upon the earth and it was reaped? A harvest is often spoken of in the Gospels; "but the workers are few." (Matt. 9:37; Luke 10:2) There, the harvest signified the gathering of the faithful; here, it signifies the end of humanity, so that if any in them were chaff, unripe and worthy of fire, it would be delivered to the fire.

Commentary on Revelation ↗

Bede

Put in your sickle, and reap, etc. Behold, he says, with iniquity abounding, the love of many has grown cold (Matt. 24), and with the heat of evil pressing, the harvest of the world has almost ceased to live. Therefore, lest the already ripe grains fall, for the sake of the elect, the days are shortened (Ibid.), and gather the tares and chaff for the flames, but store the heavenly fruit in the blessed barns (Matt. 13).

Commentary on Revelation ↗

Victorinus of Pettau

"Thrust in thy sharp sickle, and gather in the grapes of the vine," he signifies it of the nations that should perish on the advent of the Lord. And indeed in many forms he shows this same thing, as if to the dry harvest, and the seed for the coming of the Lord, and the consummation of the world, and the kingdom of Christ, and the future appearance of the kingdom of the blessed.

Commentary on the Apocalypse of the Blessed John ↗