Rev 6:9-11

THE CRY OF THE MARTYRS
You will notice that all six seals are physical attacks on human life, in contrast, with the spiritual attacks that begin the seven trumpets and bowls of wrath. The first four seals are attacks on life through famine, pestilence, war and beasts, and the fifth seal is an attack on human life through religion.
9 When he opened the fifth seal, I saw underneath the altar the souls of those who had been killed for the word of God, and for the testimony which they held.
Rev 6:9
I saw the souls – the souls of the Martyrs resurrect and reign with Christ for 1,000 years, Rev 20:4 (1471 – 2471). So, in the fifth seal, we are definitely after the year 1500.

who had been slain because of the word of God and their testimony – there actually is no word “martyr” in the Bible. The word “martus,” which means “witness,” was transformed into the word “martyr.” These are those who “overcame by the blood of the Lamb, and by the word of their testimony, and did not love their lives even when faced with death” (Rev 12:11), Here, we see the connection between “testimony” and “martyr.”
10 They cried with a loud voice, saying, “How long, Master, the holy and true, do you not judge and avenge our blood on those who dwell on the earth?” 
Rev 6:10
how long…will You refrain from judging – the “hour” of God’s judgment is described in Rev 14:7. The martyrs who resurrected during the Reformation, are crying out, for the hour of God’s judgment. The martyrdom caused by the beast would end in 1648 with the Peace of Westphalia. In Rev 8:1, we read “there was silence in heaven for about a half an hour,” approximately 267 years (half an hour), 1/24 of the 12 hour “clock” of 6,400 years. The half an hour of silence ended with the outpouring of the holy spirit in 1917.
11 There was given to each one of them a white robe. It was said to them that they should rest yet for a little time, until their fellow servants and their brothers, who would also be killed even as they were, had been fulfilled. 
Rev 6:11
given a white robe – their robes are white because they are cleansed “of the blood of the Lamb” (Rev 7:14). The Martyrs have no body, to wear a “robe.” Therefore, the robe is symbolic. The first resurrection is only for the “souls” of the Martyrs. In the last resurrection, we will all resurrect, and the bodies of the Martyrs will also be resurrected; and “we will all be changed” (1 Cor 15:51). Then, our corruptible bodies will be exchanged for incorruptible bodies. Therefore, in Rev 6:9, and Rev 20:4, John told us that he only saw “the souls” of the martyrs, because their bodies had not yet resurrected.

that they should rest a while longer – until the Lord’s Day, when the suffering of the saints comes to an end.