This clause is striking because it had already been used a few verses earlier in the proclamation of the first angel in Revelation 14:7. That angel had urged all people to fear God and give him glory,
and added this motive: “because the hour of his judgment has come.” Now in verse 15 the angel uses the identical words, with “judgment” replaced by to reap.
And that is to say that the judgment of verse 7 is carried out by the action of reaping mentioned here. The crop is ripe; of necessity the hour of judgment has come, and so reaping must commence.
15 And another angel came out of the temple, calling with a loud voice to him who sat on the cloud, “Put in your sickle, and reap, for the hour to reap has come, for the harvest of the earth is fully ripe.”