There have been many that cited these verses to validate the immortal soul doctrine and the belief that, upon death, people go to heaven. However, this description is not literal, but rather is entirely symbolic. As Scripture plainly teaches that souls are mortal, and Scripture can not be broken to sute man's vanity.
In vision, the apostle John saw before him a book or scroll sealed with seven seals. As our Moshiach opened each seal (Rev. 5:5), John was shown a preview of an event that would happen in the future (“hereafter” [4:1]). Since John was “in the spirit” as the seven seals were opened (vs. 2), the events he witnessed were not actually occurring at that time. They were heavenly previews of things that would happen later on earth.
Upon the opening of the fifth seal (Rev. 6:9), John “saw under [at the base of] the altar the souls of them that were slain.” our Moshiach had shown the meaning of the seven seals when He was on earth. He explained that the fifth seal symbolizes the coming time of Great Tribulation (Matt. 24:9-28), an event that will occur on earth.
In this vision, John was shown the future, this modern age, a time when one martyrdom has already taken place (during the Middle Ages) and a greater one (the Great Tribulation) is yet to happen. The souls who were “slain” (martyred followers of the Moshiach throughout the ages) were told, in Revelation 6:11, to “rest yet for a little season, until their fellowservants also and their brethren, that should be killed as they were, should be fulfilled.” Those who have died will continue to “rest” (remain “asleep [Eph. 5:14; I Cor. 11:30] in their graves), until others are also martyred as they were. The “souls” (dead in Moshiach) crying “avenge our blood” (vs. 10) is akin to Abel’s blood (his life [note Lev. 17:14]) crying to Elohim from the earth (Gen. 4:10). Since blood does not talk and neither do the dead (Psa. 115:17; Ecc. 9:5, 10), we understand the meaning to be symbolic, not literal. Therefore, the “souls under the altar” represent those awaiting the future martyrdom of saints.