Revelation 6:9–11 (ESV)
9 When he opened the fifth seal, I saw under the altar the souls of those who had been slain for the word of God and for the witness they had borne. 10 They cried out with a loud voice, “O Sovereign Lord, holy and true, how long before you will judge and avenge our blood on those who dwell on the earth?” 11Then they were each given a white robe and told to rest a little longer, until the number of their fellow servants and their brothers should be complete, who were to be killed as they themselves had been.
Revelation 6: 1-2: The first seal on the scroll is broken and a rider wearing a crown and carrying a bow is seen riding on a white horse. He is going out to win many battles. 3—4: The second seal is broken and a rider on a red horse appears with a mighty sword and authority to take peace from the earth so that war and slaughter is everywhere. 5-6: The third seal presents a rider on a black horse carrying a pair of scales to measure wheat and barley, etc. The idea is of scarcity of food, oil, and wine. 7—8: The fourth seal presents a rider on a pale green horse and the rider is given the name “Death,” and he has a companion named “Grave.” They are given authority to kill one-fourth of the earth with sword, famine, disease, and wild animals. 9-11: When the fifth seal is broken, under the altar John sees all who have been martyred for their testimony crying out to God for vengeance and asking how long until He judges the world for what has been done to them. They are told to wait a little longer until the “full number” of their fellow servants who will be martyred has joined them. 12-16: The sixth seal brings cataclysmic earthquakes, the sun turns black, the moon red, and all stars fall from the sky. In response all the kings and leaders of the earth hide themselves in caves and cry for the mountains to fall on them because they believe no one is able to survive.
The fifth seal speaks of Christian martyrs. While some interpretations of Revelation see them as specific groups of people (i.e. martyrs in the early church, martyrs during the tribulation period), I think the Spiritual interpretation best here: These are representative of all those who will be martyred for their faith. The question of vengeance applies as well. We leave the retaliation to God knowing that He will bring justice to us in His time. His justice, however, will be powerful and terrible as seen in the very next seal.
I will thank God that He is going to bring justice for us. I will also look for ways in which He wants to bring justice through me.