Revelation 6:9

The souls of them that were slain; the souls of the martyrs in Christ's cause represent a period of severe persecution. These are seen under the altar, which may mean either the altar of burnt-offering in the court before the temple, or the altar of incense in the outer sanctuary. If, as seems probably, the altar of burnt-offering is meant, the idea will be that they have been sacrificed on God's altar as victims in his cause, and their blood poured out beneath it. Those who understand the altar of incense, which was the symbol of intercessory prayer, explain their position from their words as recorded in verse Revelation 6:10.