Souls Under the Altar (Rev. 6: 9-11)
The quoted passage is from the Author’s Commentary on the opening of the Fifth Seal (Rev. 6:9-11). It is about the descent of Christ to Hades to preach to souls there and save the souls of the Old Testament’s righteous. The souls concerned in these passages are the souls of the Old Testament martyrs, not the New Testament’s martyrs. The reviewer questioned this out of its appropriate context.
Old Testament's Martyrs
Hebrews believed all souls of the dead go to Hades, whether they were righteous or evil. So, Jacob said to his children, “you will bring my gray hair down to Sheol [Hades] in sorrow” (Gen. 42:38, NASU).
Christ’s descent to Hades, to save the souls in Hades, was after His death on the Cross. The Author wrote:
[According to St. Peter, the Lord Jesus opened the bottomless pit, descended to Hades, and preached the gospel to its inmates by the power of the Holy Spirit. These spirits in prison were formerly disobedient in the days of Noah (1 Pet. 3:18–20).
St. Clement of Alexandria fully expounded the teaching on the descent of Christ into Hades in his Stromateis.  He argued Christ preached in Hell to the Old Testament righteous and the Gentiles who lived outside the true faith. Commenting on (1 Pet. 3:18–21), St. Clement expresses the conviction that Christ preached to all those in Hell who could believe in Christ] .
The Burnt Offering Altar
According to (Lev. 17:11), the flesh's life (the soul) is in the blood. So, the souls went under the altar when the earth opened its mouth and swallowed their blood (Gen. 4:11). On page 158, I wrote, [The souls' imagery under the altar has roots in the Old Testament. When the priest presents an animal sacrifice on the altar, the priest pours the remaining blood of the offerings at the base of the brazen altar (Lev. 4:7, 18, 25, 30).
The altar, mentioned in the passage above, is the altar of burnt offering of the old Testament. “The martyrs under the altar” are the prophets of the Old Testament, like Isaiah and others. The Lord Jesus said to the scribes and Pharisees, “Therefore you are witnesses against yourselves that you are sons of those who murdered the prophets. Fill up, then, the measure of your fathers' guilt. Serpents, brood of vipers! How can you escape the condemnation of Hell? Therefore, indeed, I send you prophets, wise men, and scribes: some of them you will kill and crucify, and some of them you will scourge in your synagogues and persecute from city to city, that on you may come all the righteous bloodshed on the earth, from the blood of righteous Abel to the blood of Zechariah, son of Berechiah, whom you murdered between the temple and the altar” (Matt. 23:31-35).
These prophets “had been slain for the word of God and for the testimony which they held” (Rev. 6:9).
The Injustice with the Martyrs
God has the Authority to send sinners to Hell. The Lord Jesus said, “do not fear those who kill the body but cannot kill the soul. But rather fear Him who can destroy both soul and body in hell” (Matt. 10:28).
God is the righteous judge, “For the Lord is righteous [just], He loves righteousness” (Ps. 11:7). “As a God of justice (Isa 30:18), He is interested in fairness and what makes for right relationships. His actions and decisions are true (Job 34:12; Rev 16:7). His demands on individuals and nations to look after victims of oppression are just demands (Ps. 82). 
The righteous demand of God is that “the wages of sin is death” (Rom. 6:23). Through Adam, sin and death and death entered the world (Rom. 5:12). Hades follows death (Rev. 6:8).
The unjust one with the martyrs is not God, but sin. Before Christ, all have sinned, “there is no one who does good, not even one.” (Rom. 3:12). They all deserved death and Hell, even the martyrs of the Old Testament. Sin made people kill the martyrs, and the killers with victims were subject to death and Hades because all had been sinners before Christ (Rom. 3:12). Sin was proven to be “exceedingly sinful” (Rom. 7:13).
Therefore, “what the law could not do in that it was weak through the flesh, God did by sending His own Son in the likeness of sinful flesh, on account of sin: He [God] condemned sin in the flesh” (Rom 8:3-4). The Lord Jesus “bore our sins in His own body on the tree, that we, having died to sins, might live for righteousness” (1 Peter 2:24). “In this is love, not that we loved God, but that He loved us and sent His Son to be the propitiation for our sins” (1 Jn. 4:10).
Now Hades and death have no authority on the souls of those who are in Christ. After the Cross of Christ, the souls of the martyrs and saints will never go to Hades. For example, the soul of Stephen. His soul was received into the hands of the Lord Jesus (Acts 7:60), not into Hades. Likewise, all the souls of martyrs in the New Testament, as St. George and all the others.
New Testament's Martyrs
The statement “we have an altar” (Heb 13:10) refers to the Altar of the New Testament and the sacrifice of Christ, the Eucharist, not animal sacrifices. Of such an altar, Isaiah prophesied, saying, “in that day there will be an altar to the Lord in the midst of the land of Egypt, and a pillar to the Lord at its border” (Isa. 19:19).
The altar of incense mentioned in Rev 8:3 belongs to the heavenly temple. In this heavenly temple, there is no need for an altar of burnt offering since atonement for our sins is now complete through the death of Jesus Christ. 
Keeping the utensils, the vestments, or relics of the martyrs and saints under the altar, in some churches, is for practical and storage purposes. It has nothing to do with imagery of the souls of the Old Testament’s martyrs under the altar of the burnt offering of the Old Testament (Rev 6:9-11).
The martyrs of the New Testament are in the front lines of the victorious. The Lord Christ promised the victorious to sit with Him on His throne. He said, “To him who overcomes I will grant to sit with Me on My throne, as I also overcame and sat down with My Father on His throne” (Rev. 3:21). They walk with Christ having white clothes. The Lord Christ also said, “They shall walk with Me in white, for they are worthy” (Rev. 3:4). The Book of Revelation presents many imageries of the victorious such as:
1- They are standing before the throne: “I looked, and behold, a great multitude which no one could number, of all nations, tribes, peoples, and tongues, standing before the throne and before the Lamb, clothed with white robes, with palm branches in their hands, and crying out with a loud voice, saying, "Salvation belongs to our God who sits on the throne, and to the Lamb!” (Rev. 7:9-10).
2- 2- They are standing on the sea of glass: “And I saw something like a sea of glass mingled with fire, and those who have the victory over the beast, over his image and over his mark and over the number of his name, standing on the sea of glass, having harps of God. They sing the song of Moses, the servant of God, and the song of the Lamb, saying: "Great and marvelous are Your works, Lord God Almighty! Just and true are Your ways, O King of the saints!” (Rev. 15:2-3).
3- 3- They are serving before the throne, and the Lamb shepherds them: “These are the ones who come out of the great tribulation and washed their robes and made them white in the blood of the Lamb. Therefore, they are before the throne of God and serve Him day and night in His temple. And He who sits on the throne will dwell among them. They shall neither hunger anymore nor thirst anymore; the sun shall not strike them, nor any heat; for the Lamb who is in the midst of the throne will shepherd them and lead them to living fountains of waters. And God will wipe away every tear from their eyes” (Rev. 7:13-17).
To order “Revealing the Father through the Book of Revelation”:
 The Stromata, Wilson, William, Clement of Alexandria (153–217).