During the first half of the 7 year Tribulation Period, two men will prophesy for 42 months (3 1/2 years). They have the power to shut up the sky from sending rain during the time of their prophesying, turn water into blood, and smite the earth with plagues (Rev. 11:6). At the end of the 42 months, the Antichrist will kill them and they will lie in the streets of Jerusalem for three and a half days. Then God will resurrect them and they will be caught up into heaven with their enemies beholding them (Rev. 11:7-12).
Many people have tried to guess their identity.
Some have said they are Moses and Elijah because of their miracles. Moses had the ability to turn water into blood (Ex. 7:17-19) and Elijah had the power to stop the rain for 3 1/2 years (James 5:17). But there's a problem with Moses. He died a long time ago (Deut. 34:6, Jude 9) and would have to be resurrected in order to do this. Since resurrected people can't be killed, this would eliminate him as a candidate.
On the other hand, Elijah is an interesting possibility. Not only did he shut up the sky from raining (1 Kings 17:1, James 5:17), he did it for the exact amount of time (three and a half years) as the prophets in Revelation 11. Coincidence?
In addition, Malachi 4:5 says, "Behold, I am going to send you Elijah the prophet before the coming of the great and terrible day of the Lord." John the Baptist went forth "in the spirit and power of Elijah" (Luke 1:17), but he himself was not Elijah (John 1:21).
Elijah was caught up into heaven without dying (2 Kings 2:9-11), so we must assume God had a reason for this. Is it possible that God has kept him preserved in his natural body in order to send him back to earth to prophesy during the Tribulation Period? That's a definite possibility.
The only other person who has never died is Enoch. "By faith Enoch was taken up so that he should not see death; and he was not found because God took him up; for he obtained the witness that before his being taken up he was pleasing to God" (Heb. 11:5). Is it possible that he is the other prophet?
An interesting writing from the 4th century A.D., the gospel of Nicodemus (which is not scripture) reads, "I am Enoch who pleased God, and was translated by him. And this is Elijah the Tishbite. We are also to live to the end of the age; but then we are about to be sent by God to resist Antichrist, and be slain by him, and to rise after three days, and to be caught up in the clouds to meet the Lord." Again, this is not scripture, but it does give an insight into what some people believed in the 4th century concerning the identity of the two witnesses.
We believe the 2 witnesses are Enoch and Elijah, although they could also be two people who are saved after the Rapture. They will prophesy during the first 3 1/2 years of the Tribulation because after they are killed, they lie in the streets of Jerusalem for over 3 days. If they prophesied during the last 3 1/2 years, those 3 days would be after the Second Coming. That doesn't fit, so their prophesying has to be during the first half of the Tribulation.
So here's the scenario. The two witnesses prophesy for 42 months, bringing plagues upon the earth, and no one is able to kill them (Rev. 11:5). On the exact same day, the Antichrist goes into the temple in Jerusalem and stops the sacrifices (Dan. 9:27). Then he takes his seat in the temple and declares himself to be God (2 Thess. 2:3-4). He walks out of the temple and immediately meets the two prophets, who have just finished their testimony. No one on earth has been able kill the two witnesses for 42 months, but because their ministry is finished, the Antichrist is able to kill them. (Rev. 11:7). This gives him instant credibility in the world's eyes as the most powerful man on earth. After lying dead in the streets of Jerusalem for three and a half days, the two prophets suddenly are resurrected and ascend into heaven.
Some people have used this passage as proof as a "Mid-Trib" Rapture, but this can't be the case for three reasons:
First, because these two are the only ones resurrected, not the entire Church.
Second, the Rapture happens quickly in the twinkling of an eye (1 Cor. 15:52), but these two slowly ascend into heaven with people watching (Rev. 11:12).
Third, this occurs three and a half days after the middle of the Tribulation. This is not the Rapture, but a special resurrection and ascension for only these two people.