"And when He broke the fifth seal, I saw underneath the altar the souls of those who had been slain because of the word of God, and because of the testimony which they had maintained; and they cried out with a loud voice, saying , 'How long, O Lord, holy and true, wilt Thou refrain from judging and avenging our blood on those who dwell on the earth?' And there was given to each of them a white robe; and they were told that they should test a little while longer, until the number of their fellow servants and their brethren who were to be killed even as they had been, should be completed also". (Rev. 6:9-11 NASB).
These are the martyrs that have been killed for their faith in Christ during the persecution of the forth seal (Matt. 24:9-14) and are now being killed during the "great tribulation (Matt. 24:21, 22)." These are truly blessed (Rev. 16:13) who are the believing of Israel and "the rest of her offspring" (Rev. 12:17), the true church, both make up the body of Christ at this time (Gal. 3:28; Col. 3:11; Eph. 5:30, 32). The cry, "How long", means the great tribulation is still going on "until the number of their fellow servants (the Church) and their brethren (believing Jews)" are "beheaded because of the testimony of Jesus and because of the word of God, and those who had not worshipped the beast and his image, and had not received the mark upon their forehead and upon their hand" (Rev. 20:4 NASB). Note: they are "given" white robes as encouragement, not yet to be put on until the resurrection (and rapture) when they receive their glorified bodies (1 Cor. 15:35-58).