It is commonly assumed that the souls John sees under the altar (v. 9) are in heaven because the altar is assumed to be the altar in God's heavenly temple. When we read on in the book, we discover that this assumption is reasonable, since "the altar" in heaven is mentioned again in 8:3 (twice), 8:5, 9:13, 14:18 and 16:7. What is odd is that in describing the fifth seal John speaks of the altar (with a definite article) as if it is well-known to his readers, even though he now introduces it for the first time. No altar was included in his detailed description of the heavenly throne room scene in chapters 4-5. The closest he came to mentioning an "altar" (Greek thysiasthrion) was his reference to "golden bowls of incense" (Greek thymiamata), representing as he said "the prayers of the saints" (5:8). Another odd thing is the phrase itself: under the altar. Who are these souls of those who had been slain because of the word of God and the testimony they had maintained (v. 9), and why are they under the heavenly altar? There is general agreement that they are martyrs. Even though no specific enemies or forces of evil have been named in chapters 4-6, the stubborn fact of martyrdom proves that such enemies do exist and sooner or later must be faced.
Elsewhere in the book of Revelation the testimony associated with martyrs is called "the testimony of Jesus" (1:9; compare 12:17; 17:6; 19:10; 20:4), but here it is more general: the testimony they had maintained. Because martyrdom was not yet a widespread experience in the Christian movement, this first vision of martyrs is probably meant to include Old Testament and Jewish, as well as Christian, martyrs (for example, those put to death in the time of the Maccabees; see 2 Macc 6:7--7:42; 4 Macc 5:1--6:30; 8:3--12:19; Heb 11:32-38). The assumption behind the visions is that those who were to be killed as they had been (v. 11), or (as stated in 14:13) "the dead who die in the Lord from now on," would be Christians martyred specifically for "the testimony of Jesus."
Many commentators find in the fifth seal an allusion to Leviticus, where the blood of a sacrificed bull was poured out "at the base of the altar of burnt offering" (Lev 4:7), with the understanding that "Since life was thought to be in the blood of the animals, and of humans (Lev 17:11, 14), and since `life,' `soul,' `self,' were interchangeable terms, the lives or selves of sacrificed victims could be thought of as being at the base of or `under' the altar" (Boring 1989:125). But would John have said under the altar when he meant "at the base of the altar"? If he did not want to use the exact words of Leviticus, why would he not have written "before" or "in front" of the altar (Greek enopion) in the same way he described earlier what was "before" the throne (4:5-6)?
The preposition translated under (Greek hypokato) is a strong one, meaning "below" or "beneath" (compare the phrase "under the earth," in distinction from "in heaven" or "on earth," in 5:3, 13). John's choice of words raises the distinct possibility that the souls he saw under the altar were not in heaven at all, but far below it on earth--the same earth that had just been scarred by the disasters of the first four seals. John could have written "under heaven" (in the sense of "on earth," as in Gen 6:17; 7:19) or "under the throne." Instead he wrote under the altar, probably because the vision centered on what he calls elsewhere "the prayers of the saints" (5:8; 8:3-4). Not the souls themselves but their prayers are the sacrifices that ascend like incense from earth to heaven--from "under the altar" to the altar itself.
These souls are not disembodied spirits. They are, after all, visible to John. Nor are they the "lives" or "selves" of slaughtered victims as a kind of abstraction, nor are they typical of what theologians like to call "the intermediate state" (the interval between a believer's physical death and the final resurrection). Rather, at least within the horizons of John's vision, these souls are people with voices and real bodies, like the "beheaded" souls of 20:4. They are martyrs, not just in the sense of bearing testimony (Greek martyria, v. 9), but in the sense of having been "killed" (v. 11) for their testimony. Like Abel, the first martyr, who "still speaks, even though he is dead" (Heb 11:4; compare Gen 4:10), they cry out for justice to be done. Their prayer (v. 10) is the heart of the fifth seal. It is the prime example of what was meant by the "prayers of the saints" (5:8; compare 8:3-4).