Revelation 14:15-20
(Rev 14:15 KJV)
And another angel came out of the temple, crying with a loud voice to him that sat on the cloud, Thrust in thy sickle, and reap: for the time is come for thee to reap; for the harvest of the earth is ripe.

Here an angel now approaches the Lord Jesus Christ and commands Him to put in the sickle for the harvest is ripe. Since no angel ever has any authority over the Lord Jesus Christ, this angel is no doubt bringing the command from God the Father and he is just relaying it.

(John 6:37 KJV) All that the Father giveth me shall come to me; and him that cometh to me I will in no wise cast out.

John 6:37 states that all that the Father gives Jesus will come to Him. Now there is going to come a time right at the end when the last one whom the Father named will be brought to the Lord Jesus Christ and then comes the harvest of the earth. Now this harvest does not only include the Elect of God, the unbelievers also will be harvested from the earth and will stand before the Lord Jesus Christ.
(Mat 25:31-32 KJV) When the Son of man shall come in his glory, and all the holy angels with him, then shall he sit upon the throne of his glory: {32} And before him shall be gathered all nations: and he shall separate them one from another, as a shepherd divideth his sheep from the goats: So here we have the sickle being cast into the harvest and all things in the fields will be harvested. (Mat 13:38-39 KJV) The field is the world; the good seed are the children of the kingdom; but the tares are the children of the wicked one; {39} The enemy that sowed them is the devil; the harvest is the end of the world; and the reapers are the angels. Matthew 13:38-39 teach us what the field is and what the harvest is so there can be no guesswork.

(Rev 14:16 KJV)
And he that sat on the cloud thrust in his sickle on the earth; and the earth was reaped.

Everything has an order, even the reaping of the harvest of the earth. In verse 15 we saw the generic harvest and now we are seeing the specific order of that harvest.
(1 Th 4:16-17 KJV) For the Lord himself shall descend from heaven with a shout, with the voice of the archangel, and with the trump of God: and the dead in Christ shall rise first: {17} Then we which are alive and remain shall be caught up together with them in the clouds, to meet the Lord in the air: and so shall we ever be with the Lord.  1 Thessalonians 4:16-17 teaches us that the Lord is going to raise the dead in Christ and then those who are still alive will be brought to Heaven on their heels. This will be the rapture of the Saints into Glory.

(Rev 14:17 KJV)
And another angel came out of the temple which is in heaven, he also having a sharp sickle.

Now we are reading that another angel comes out of the temple of heaven and this angel also has a sharp sickle. His responsibility will now be to bring the unbelievers into judgment. The believers have all been brought to heaven and now it is the resurrection and judgment of the nations that we saw in Matthew 25. This will be the judgment upon Babylon which is the universal kingdom of Satan.

(Rev 14:18 KJV)
And another angel came out from the altar, which had power over fire; and cried with a loud cry to him that had the sharp sickle, saying, Thrust in thy sharp sickle, and gather the clusters of the vine of the earth; for her grapes are fully ripe.

This angel has authority over fire. When we look in the Scriptures we will find that fire has to do with judgment.
(Mat 18:8 KJV) Wherefore if thy hand or thy foot offend thee, cut them off, and cast them from thee: it is better for thee to enter into life halt or maimed, rather than having two hands or two feet to be cast into everlasting fire.  Now this angel tells the angel in the previous verse to thrust in the sharp sickle because the grapes are fully ripe. The principle for this is found back in Genesis 15:16. (Gen 15:16 KJV) But in the fourth generation they shall come hither again: for the iniquity of the Amorites is not yet full. God told Abraham that the nation of Israel would come out of Egypt in the fourth generation because the iniquity of the Amorites was not yet full. In 14:18, we are told that the grapes are fully ripe which means the iniquity of the earth is now full and ready for judgment.

(Rev 14:19 KJV)
And the angel thrust in his sickle into the earth, and gathered the vine of the earth, and cast it into the great winepress of the wrath of God.

The angel now puts forth the sickle and gathers the vine of the earth into the great winepress of God. This simply means that all the unbelievers are now readied for judgment and are in the process of being judged at the Great White Throne Judgment. In Deuteronomy 32:31-33 we read that God is making a separation between His people and those of the world. He likens the vine of the world to that of Sodom and Gomorrah and the fruit of that vine, the wine, as poison of serpents and cruel venom of asps which are poisonous snakes. Of course, Satan has been dubbed a serpent and spews his venom through the unbelievers who attack the church as we have been seeing so far in Revelation.

(Deu 32:31-33 KJV) For their rock is not as our Rock, even our enemies themselves being judges. {32} For their vine is of the vine of Sodom, and of the fields of Gomorrah: their grapes are grapes of gall, their clusters are bitter: {33} Their wine is the poison of dragons, and the cruel venom of asps.

(Rev 14:20 KJV)
And the winepress was trodden without the city, and blood came out of the winepress, even unto the horse bridles, by the space of a thousand and six hundred furlongs.

Here we are reading a very gruesome description of the judgment of the unbelievers. First, we are told that this judgment is done outside the city. This would allude to the city which is the body of believers.
(Rev 21:10 KJV) And he carried me away in the spirit to a great and high mountain, and showed me that great city, the holy Jerusalem, descending out of heaven from God, Our judgment was paid for by Christ and therefore we do not stand for any type of judgment since all our sins have been removed and paid for.

The judgment is being graphically described as being a great bloodletting judgment. The 1600 furlongs would equate to 200 miles. Since the whole world is being judged at this point, this number is symbolic and it would be easier if it was looked at as 4 X 400. 4 is the number of universality which means that the world on judgment day is being symbolically depicted as a great blood bath which would be universal. The bridle of a horse is approximately 5 feet off the ground but varies as the horse walks. This too would be a symbolic description that the judgment will be so intense that the blood will cover the earth up to five feet in height. Of course, this is symbolic to show that the judgment is universal and awful. The winepress is symbolic for the Great White Throne Judgment where no unbeliever will escape and that is why the symbol of blood is universal. There are some who try to make this verse literal but it will not stand the scrutiny and the context of this chapter.