And another angel came out from the altar, which had power over fire. Both in Revelation 6:9 and Revelation 8:3 the altar is connected with judgment. The angel here described is he who is referred to in those places, the fire being the fire of the altar, the fire of judgment (Revelation 8:3), or, less probably; the angel who has power over fire generally (as Revelation 7:1; Revelation 16:5).
And cried with a loud cry to him that had the sharp sickle, saying. Again the "loud voice," characteristic of the heavenly utterances (cf. verse 15, etc.). Thrust in thy sharp sickle, and gather the clusters of the vine of the earth; for her grapes are fully ripe; send forth thy sharp, etc.
(see on verse 16). The sickle is figurative of the instrument by which the career of those on earth is terminated. The "sickle" and the "wine press" are both alluded to in the passage quoted above (on verse 14) from Joel 3:13.
(For the meaning of this gathering of the vintage, as representing the punishment of the wicked, see on Joel 3:16.