Revelation 6:11 says, "And to each of them was given a white robe; and it was said to them that they should rest yet a little while, until the number of their fellow slaves and their brothers who were about to be killed even as they were should be completed also." "Yet a little while" signifies that the martyrs' cry should take place near the end of the present age.
A white robe signifies that the martyrdom of the martyrs was approved by the Lord, and rest signifies that they are resting in paradise.
"Until the number of their fellow slaves and their brothers who were about to be killed...should be completed" indicates the martyrdom during the great tribulation (Rev. 20:4). Many believers will be martyred by the Antichrist in those three and a half years.
The sixth seal is the shaking of the earth and heaven (Rev. 6:12-17).
There will be a great earthquake, the sun will become black, the whole moon will become as blood, the stars will fall to the earth, the heaven will recoil, and every mountain and island will be moved (Rev. 6:12-14).
The kings, the great men, the captains, the rich and the strong, and every slave and free man will hide themselves in the caves and in the rocks of the mountains, saying to the mountains and rocks, "Fall on us and hide us from the face of Him who sits upon the throne and from the wrath of the Lamb; for the great day of Their wrath has come, and who is able to stand?" (Rev. 6:16-17).