And when He opened the fifth seal, I saw under the altar the souls of those who had been slain because of the word of God and because of the testimony they had. And they cried with a loud voice: “How long, O Lord, holy and true, will You not judge and avenge our blood upon those who dwell on the earth?” And to each of them was given a white garment, and it was said to them that they should rest for a little while yet, until their fellow servants, that is, their brothers who are about to be killed, might be made complete as they themselves had been. Revelation 6: 9-11

This imagery of faithful martyrs under the altar is based on the Old Testament sacrificial ritual at the temple. There, at the altar of burnt offering, the sacrificial animal would be burnt. Its blood would be drained and poured out at the bottom of the altar. That’s why these faithful saints are pictured beneath the altar. Their life-blood had been poured out as an offering to God. This seal represents those persecuted under the previous four seals, especially the second one.

Also, referencing the Old Testament, the altar of burnt offering was located in the temple’s outer court. In view of that, the altar that John saw was not in the heavenly temple but on earth; earth being the symbolic outer court.

It should be understood that there were not literal people underneath the altar because people can’t fit beneath an altar. They were figurative images of people whose blood was underneath the altar. That the “How long” question was voiced from underneath, is akin to the symbolism of Abel’s blood crying out from the ground to God because of his death (Genesis 4: 10).

Revelation’s “How long” question is in terms of when will justice be served and God’s people be vindicated. The question is related to the “How long” question asked and answered in Daniel 12: 6, 7. Many scholars apply Daniel’s “How long” question to the great tribulation that occurred during the Middle Ages. If so, it is likely that the martyrs that John saw underneath the altar came from that period.

White garments were given to the martyrs, for they were victorious overcomers due to their faith in and witness of the gospel. They were then told, in so many words, to maintain that faith and to rest a little while longer. Why? Because it wasn’t time for the resurrection. You see, in actuality, these martyred saints are still in their graves on earth.

Others of God’s people will be killed before Christ returns, particularly those who will not worship the beast, nor his image, nor take on the mark of the beast. At Jesus’ return, the “How long” question will be put to rest. Justice will then be carried out against the wicked and the persecuting enemies of God’s people.