Under the altar (hupokatō tou thusiastēriou). “Under” (hupokatō), for the blood of the sacrifices was poured at the bottom of the altar (Lev 4:7). The altar of sacrifice (Exo 39:39; Exo 40:29), not of incense. The imagery, as in Hebrews, is from the tabernacle. For the word see Mat 5:23., often in Rev (Rev 8:3, Rev 8:5; Rev 9:13; Rev 11:1; Rev 14:18; Rev 16:7). This altar in heaven is symbolic, of course, the antitype for the tabernacle altar (Heb 8:5). The Lamb was slain (Rev 5:6, Rev 5:9, Rev 5:12) and these martyrs have followed the example of their Lord.
The souls (tas psuchas). The lives, for the life is in the blood (Lev 17:11), were given for Christ (Phi 2:17; 2Ti 4:6).
Of the slain (tōn esphagmenōn). See Rev 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 (dia ton logon tou theou). As in Rev 1:9, the confession of loyalty to Christ as opposed to emperor-worship.
And for the testimony which they held (kai dia tēn marturian hēn eichon). See also Rev 1:9. Probably kai equals “even” here, explaining the preceding. The imperfect tense eichon suits the repetition of the witness to Christ and the consequent death.