A.T. Robertson Commentary Revelation 6:9

"And when he opened the fifth seal, I saw underneath the altar the souls of them that had been slain for the word of God, and for the testimony which they held:" — Revelation 6:9 (ASV)

Under the altar (υποκατω του θυσιαστηριου). "Under" (υποκατω), for the blood of the sacrifices was poured at the bottom of the altar (Leviticus 4:7). The altar of sacrifice (Exodus 39:39; Exodus 40:29), not of incense. The imagery, as in Hebrews, is from the tabernacle. For the word see Mt 5:23f. , often in Rev. (Revelation 8:3,5; 9:13; 11:1; 14:18; 16:7). This altar in heaven is symbolic, of course, the antitype for the tabernacle altar (Hebrews 8:5). The Lamb was slain (5:6,9,12) and these martyrs have followed the example of their Lord.

The souls (τας ψυχας). The lives, for the life is in the blood (Leviticus 17:11), were given for Christ (2 Timothy 4:6).

Of the slain (των εσφαγμενων). See 5:6. Christians were slain during the Neronian persecution and now again under Domitian. A long line of martyrs has followed.

For the word of God (δια τον λογον του θεου). As in 1:9, the confession of loyalty to Christ as opposed to emperor-worship.

And for the testimony which they held (κα δια την μαρτυριαν ην ειχον). See also 1:9. Probably κα equals "even" here, explaining the preceding. The imperfect tense ειχον suits the repetition of the witness to Christ and the consequent death.