The Martyrs underneath  the Altar (the fifth seal), Revelation Chapter 6 Verses 9 through 11 (NASB)


     And when He broke the FIFTH SEAL,
       I saw underneath the ALTAR the SOULS
       of those who had been SLAIN
These are most commonly believed to be those martyred during the tribulation period but it may include ALL martyrs since Christ's resurrection.
  1. because of the Word of God, and
  2. because of the TESTIMONY which they had maintained;
This is John's expression for true believers. He uses it for himself, Revelaion 1:9 and for the overcomers, Revelation 12:10-11.

The expression "under the alter" places this part of the vision in heaven. Recall that the four horsemen under the previous four seals were sent to the earth. There is a symbolic parallel in the Old Testament sacrifices (and the blood related to that) in Leviticus 17:8-16.

It is clear from the use of "the Word of God" and "testimony" that belivers will not be killed because of their 'title' such as 'I am a Christian' or 'I am a church member at ...'. These martyrs MUST HAVE KNOWN the Word of God, and they did TESTIFY to Christ's redeeming power. 

It is for this reason, the TESTIMONY, that the devil will go to great length's to keep humans from learning, knowing, and understanding the Word of God, both inside and outside the church.
Just because the believers are removed in the uptaking, does not mean that there may not be others who will read the Word from the 1000s of Bibles left behind, video tapes left behind, and even this exposition which will be left behind. Even some of the false prophets and false teachers may realize, repent, and believe; these may be among the martyrs.
     and they cried out with a loud voice, saying
These are those souls under the alter in verse 9 who were killed for the Word and their testimony.
     "How long, O LORD,
This term, O LORD, denotes the Soverign Lord 
     holy and true,
This is the same description of Jesus given by the angel to the church in Philadelphia, Revelation 3:7.
   wilt Thou refrain from judging and avenging our blood
     on those who dwell on the earth?
This is not earthly hateful revenge. These souls are in heaven, pure and bought, BUT God's righteousness demands evil to be judged.  The curious thing about this portion is that, even in heaven, the souls do NOT know when God will perform this vengence. This is to say, both heaven occupants and earth occupants do NOT know when these seals will be broken. We do know, however, that God's timing is always perfect!
     And there was given to each of them a white robe;
The white robes are symbolic of their righteous deeds and imputed righteousness by their faith in Christ. We see these souls again in Revelation 7:9. Here they are given their robes; in chapter 7, they are standing around the throne in their white robes giving honor and praise to the One whose blood 'washed their robes', Revelation 7:14.

From physics, white light is the combination of all the colors of light; in a sense, white denotes all radiated energy. This indicates that heaven, since white is the prevalent color, may be a very large concentration of energy. We also know that when material is in close proximity to a source of energy that some of that energy is gained by the material. An example of this is heating a rod of iron; the longer it stays in "the fire" the "whiter" it glows.

So, too, after Moses came back to earth after talking with God, he had to place a veil over his face to not frighten the people, Exodus 34:29-35. When Christ was transfigured before the three disciples, Mark 9:2-8, Christ's garments became radiant and "exceedingly white". Everyone who has had a glimpse of the radiance and majesty comes away TOTALLY changed (2 Peter 1:16-18). Are you looking forward to an encounter like this?

     and they were told
     that they should rest a little while longer,
What is time to an eternal God? So a little while longer may be a long time for us.
     UNTIL - the number of
  • their fellow servants and
  • saved Gentiles, multitudes of all nations (Revelation 7:9-17)

  • their brethren who were to be killed even as they had been,
  • saved Jews, offspring of the woman (Revelation 12:16-17)
    should be completed also.

Everything has a time and place in God's timetable. And the recompensing of deeds, 2 Corinthians 5:1-10, whether good or bad WILL take place!
"Scripture taken from the NEW AMERICAN STANDARD BIBLE®, © Copyright The Lockman Foundation, 1960, 1962, 1963, 1968, 1971, 1972, 1973, 1975, 1977, 1995 Used by permission."

References to the Revelation of Jesus Christ are in bold italics. All other references are in bold only.