At the start of the 1st half of Tribulation, two individuals will preach the gospel of Jesus Christ, teach prophecy, condemn the Antichrist and False Prophet, and perform miracles in the city of Jerusalem. They will be allowed 1260 days (3 ½ years) to do so. The scriptures of Revelation 11: 3-6 outline their work.
Revelation 11: 3-6
3 And I will give power to my two witnesses, and they will prophesy for 1,260 days, clothed in sackcloth." 4 These are the two olive trees and the two lamp stands that stand before the Lord of the earth. 5 If anyone tries to harm them, fire comes from their mouths and devours their enemies. This is how anyone who wants to harm them must die. 6 These men have power to shut up the sky so that it will not rain during the time they are prophesying; and they have power to turn the waters into blood and to strike the earth with every kind of plague as often as they want.
It’s clear the Two Witnesses will not be your average evangelists. They will be given by God supernatural powers to protect themselves with fire from their mouths as well as control over environmental conditions and the ability to bring plagues against the world whenever they want.
Their choice of clothing, sack cloths, is interesting and highly symbolic. Sack cloths could indicate humbleness, simplicity or an ominous warning to the non-Christians left behind of the coming danger, distress, humiliation and sorrow that is about to be unleashed by God’s Judgments, Antichrist and False Prophet.
The Men and Their Mission
There are a many theories by several respected Biblical scholars as to whom these witnesses might be. As they are adorned with sack cloths, as the prophets of the Old Testament, most scholars believe they will be two of the following Old Testament Prophets; either Moses, and Elijah or Enoch.
Moses, a prophet who performed many God-given supernatural feats during the Jewish exodus from Egypt; feats such as turning water into blood as well as multiple plagues against Egypt and Elijah, who called fire down from Heaven and kept it from raining, appear to be the leading candidates to fill the roles of the two witnesses. Malachi 4:5 give further evidence that Elijah will be one of The Two Witnesses.
"See, I will send you the prophet Elijah before that great and dreadful day of the Lord comes.”
Although God will give The Two Witnesses the ability to protect themselves supernaturally, their time on Earth will be finite and has already been predetermined by God to be only 3 ½ years. After their pre-determined 3 ½ years on Earth prophesying, God will remove his protection and they will be killed by the Antichrist.
Now when they have finished their testimony, the beast that comes up from the Abyss will attack them, and overpower and kill them.
After their death, the bodies of the two witnesses will be left unburied on the streets of Jerusalem for 3½ days as a macabre and ominous warning for all to see. In Biblical time, leaving a body unburied like this was one of the worst means of defiling and disrespecting someone who has died. In response to their murder, which brings an end to the droughts and plagues caused by the witnesses, their slayings will be greeted with much rejoicing throughout the world. The very clear picture we have here is of two Old Testament prophets sent back to Earth by God who are ridiculed, spat upon, killed and defiled by the very people they were assigned to minister to (Revelation 11:10).
Their bodies will lie in the street of the great city, which is figuratively called Sodom and Egypt, where also their Lord was crucified. For three and a half days men from every people, tribe, language and nation will gaze on their bodies and refuse them burial. The inhabitants of the earth will gloat over them and will celebrate by sending each other gifts, because these two prophets had tormented those who live on the earth.
Just like Jesus, in the same city of Jerusalem, the two witnesses will be raised from the dead. Their mission completed. The ascension of the two witnesses into Heaven will be happen publically in front of crowds of people. One can assume, in today’s age of instant communication via satellite TV and internet, the resurrection and ascension of these two witnesses will be seen by millions and millions all over the world.
But after the three and a half days a breath of life from God entered them, and they stood on their feet, and terror struck those who saw them. Then they heard a loud voice from heaven saying to them, "Come up here." And they went up to heaven in a cloud, while their enemies looked on.
Soon after the rapture of the two witnesses into Heaven, a terrible earthquake will destroy 1/10 of Jerusalem and 7,000 people will be killed. The survivors who were not killed will turn to God for forgiveness. Their repentance comes near the end of the bowl judgments, offering encouragement in the midst of the world’s greatest horror.
At that very hour there was a severe earthquake and a tenth of the city collapsed. Seven thousand people were killed in the earthquake, and the survivors were terrified and gave glory to the God of heaven.
Charles Swindoll: Daniel, Volume 2: God’s Plan for the Future
Charles Swindoll: Revelation—Unveiling the End
Tim LaHaye and Ed Hindson: The Popular Encyclopedia of Bible Prophecy