In heaven John sees the prototype of which the tabernacle that Israel built (in Moses’ day) was a copy (see Hebrews 8:5; Exodus 25:40). The tabernacle of the Old Testament knew two altars. The bronze altar stood outside in the courtyard. On this altar sacrifices were made on a daily basis to atone for Israel’s sins (see Exodus 27:1–8, with details of sacrifices in Leviticus 1:1 – 7:38). In the Holy Place inside the tabernacle stood a golden altar on which incense was to be offered morning and evening (Exodus 30:1–10). The incense offered on this altar represented the prayers of the people (see Psalm 141:2; Luke 1:10). As the aroma of the incense penetrated to the throne of God behind the veil in the Most Holy Place, so the prayers of God’s people penetrated to God himself on his heavenly throne. According to Revelation 6:10, the souls John saw under the altar were crying out to God, that is, praying. Thus, it makes the most sense to understand the altar John references here as the altar of incense. The same altar also appears in Revelation 8:3–5.
9 When he opened the fifth seal, I saw under the altar the souls of those who had been slain for the word of God and for the witness they had borne.