Revelation 14 is again LOADED with symbolism. If you don't have a good understanding of the symbolism and language from the Old Testament scriptures, chances are you will miss that much of it comes from there. This will lead to great confusion. But Revelation 14 reads..
" And another angel came out of the temple, crying with a loud voice to Him who sat on the cloud, “Thrust in Your sickle and reap, for the time has come for You to reap, for the harvest of the earth is ripe.” So He who sat on the cloud thrust in His sickle on the earth, and the earth was reaped...
Then another angel came out of the temple which is in heaven, he also having a sharp sickle. And another angel came out from the altar, who had power over fire, and he cried with a loud cry to him who had the sharp sickle, saying, “Thrust in your sharp sickle and gather the clusters of the vine of the earth, for her grapes are fully ripe.” So the angel thrust his sickle into the earth and gathered the vine of the earth, and threw it into the great winepress of the wrath of God." Revelation 14:14-19
A sickle was an ancient tool that was very sharp and was used for harvesting grain. In the bible it often represents fierce judgment. The "grapes are fully ripe" simply meant that those whom He was going to judge, were ready for judgement. The time had arrived.
We also heard a parable from Jesus about a sickle and reaping the harvest in Mark chapter 4. It reads...
"And He said, “The kingdom of God is as if a man should scatter seed on the ground, and should sleep by night and rise by day, and the seed should sprout and grow, he himself does not know how. For the earth yields crops by itself: first the blade, then the head, after that the full grain in the head. But when the grain ripens, immediately he puts in the sickle, because the harvest has come.”
So we see that the Kingdom of God was going to come at the time that this sickle was "put in" (or "thrust in") because the harvest was ready. The sickle and the reaping of the harvest are linked to the time of the Kingdom of God. Now let's see when that Kingdom would come according to Jesus...
“Assuredly, I say to you, there are some standing here who shall not taste death till they see the Son of Man coming in His kingdom.” Matthew 16:28
So the Kingdom was ASSURED to be coming BEFORE some of the men standing in front of Jesus would die. Therefore, this picture in Revelation 14 is no doubt, a past event.