Revelation 6: 9 - 11: And when he had opened the fifth seal, I saw under the altar the souls of them that were slain for the word of God, and for the testimony which they held: And they cried with a loud voice, saying, How long, O Lord, holy and true, dost thou not judge and avenge our blood on them that dwell on the earth? And white robes were given unto every one of them; and it was said unto them, that they should rest yet for a little season, until their fellowservants also and their brethren, that should be killed as they were, should be fulfilled.
The Old Testament tabernacle had two altars; the first was the Brazen Altar (of sacrifice) in the Outer Court, and the second was the Golden Altar (of incense) in the Holy Place.
The souls under the altar described in Revelation 6: 9 - 11 are the Jews who were persecuted and killed during the seven church ages because their forefathers rejected Jesus; Matthew 27: 25: Then answered all the people, and said, His blood be on us, and on our children.
These Jews are crying out for vengeance, whereas a Christian would ask forgiveness for those that persecute them; Luke 12: 34: Then said Jesus, Father, forgive them; for they know not what they do. And they parted his raiment, and cast lots.
The souls of the fifth seal Jews are in a holding place (represented by the Brazen Altar) beyond this world; they will be given robes in the form of theophany bodes (Revelation 6: 11) at the Great White Throne of Judgment.
The Christians who have died during the seven church ages are beyond the Curtain of Time in the Region of the Blessed which is represented by the Golden Altar; these Christians are already robed with theophany bodies and will receive their glorified bodies at the first resurrection (1 Thessalonians 4: 16 - 18).