In verses 1,2 the Altar, the Temple and the Worshippers have been measured and found wanting. Israel is not ready for the kingdom and the blessing to be bestowed on them.
Verses 1,2 deal with the first half of the tribulation period, verse 3 deals also with the first half of the tribulation period. "And I will give power unto my two witnesses, and they shall prophesy (preach) a thousand two hundred and threescore days, ..." The Lord sends two witnesses to Jerusalem to preach the gospel of the kingdom. The means of salvation is always the same, the method of savation never changes, it is always by repentance and faith in Gods' provision for sin, the sacrificial death of Christ and the shedding of His blood.
However the blessings of salvation are different, for the Christian, it is to be forever with Christ in heaven, for Israel it is to be a part of the earthly millenial kingdom, for the tribulation saints, it is to be before the throne of God forever (Revelation 7:13-17).
The question always arises as to who the two witnesses are?
Some have suggested that one of the witnesses is Enoch because he never experienced physical death. Genesis 5:24 says, "Enoch walked with God: and he was not; for God took him". However Enoch was a gentile and these two witnesses are obviously Israelites, Enoch does not meet the qualifications. It seems to me that the obvious choice would be Elijah and Moses. Malachi 4:5-6 says that Elijah will come before the Day of the Lord. The most convincing evidence is the appearance of Moses and Elijah on the mount of transfiguration, talking with Christ, and sharing in the glory of the coming kingdom.
Verse 5, we see the two witnesses calling down fire from heaven to kill those who would attempt to kill them, Elijah called down fire from heaven to kill those who sought to kill him. Verse 6, we see them shutting the heaven that it rain not, that is exactly what Elijah did. They evidently produce signs and wonders even as Moses did. We see them smiting the earth with "all plagues" even as Moses did.
Notice their message, in verse 3, they preach, "clothed in sackcloth", every Israeli knows that this is the garment of grief and repentance. They are calling for repentance in an hour of great wickedness.
We need to be constantly reminded that God hates sin with an incredible and eternal hatred, we look into the eternal fires of hell to see the awful hatred of God for sin. We look to calvary to see God's amazing mercy and grace, we are also reminded that God's mercy and grace is conditioned upon repentance and faith, faith that demonstrates its validity by actions. Jesus said to the Pharisees, "Bring forth works meet for repentance".
Here we see both God's mercy and God's anger at sin in the ministries of the two witnesses. We live in the age of grace, a time when God delays judgment for sin, the tribulation period will be time when sin will be directly and immediately judged by God. Shortly after the mid-tribulation point two thirds or more of the worlds population will have been killed for their sinfulness. Those who refuse to accept that God is God of Judgment have never read the bible, the bible is filled with judgment from cover to cover. Those who do not believe that God would send men and women to an eternal hell will find out when it will be forever too late.