When Jesus opened the fifth seal, John’s attention was drawn toward the Altar of Incense, where the prayers of the saints were offered to God by the High Priest. There John was given a vision of those who were martyred for the testimony they bore, concerning the Word (Logos, G3056) of God (Revelation 6:9). Martyr vindication is a major theme found throughout the Bible (Matthew 23:29-33, 35-36), and it is this very matter that is referred to in the question the souls asked the Lord.
Notice that John heard the prayer of the martyred saints: “How long, O Lord, holy and true, do you not judge and avenge our blood on them that dwell on the earth” (Revelation 6:10)? This prayer is based upon Psalm 79, and their prayers are brought to bear upon the judgment seat of Christ. They cry out for justice (vindication), because the wicked have invaded the inheritance of Christ (Matthew 21:38; 23:34; Ephesians 1:18). They have defiled his Temple (cp. 1Corinthians 3:16-17) and laid waste the city of God, Jerusalem (Revelation 21:2; cf. Acts 8:1-3; Psalm 79:1-4).
“How long, Oh Lord…” (Revelation 6:10; Psalm 79:5) will the wicked go unpunished? The wicked don’t know the Lord, nor do they have any desire to know him (Romans 1:28-32). Not only don’t they call upon his name, but they devour and destroy his dwelling place (Psalm 79:6-7; Acts 8:1-3; 1Corinthians 3:17). The saints cry out for mercy, because they are weak and persecuted, asking for forgiveness of their own transgressions, so that the name of God could be glorified in them (Psalm 79:8-9; cf. Revelation 6:11; 7:9, 14).
Why should the wicked say of the righteous: “Where is their God?” (Psalm 79:10; cf. Matthew 27:43). Therefore, the cry of the persecuted is to let the name of God be praised through the vindication of the martyrs. Thus, by hearing the prayers of these righteous ones and by revenging their blood upon the heads of their enemies, God will be known to the ignorant, and he will be admired in those who have shown they love him (Psalm 79: 11-13; 2Thessalonians 1:10).
The righteous looked for and longed for vindication. They longed for the Lord to sit in judgment against the wrongdoing done by evil men in high places of authority. This was not a matter of forgiving a wrong committed against one or two people (cf. Luke 23:44: Acts 7:60). It was a cry for justice and setting things right on a national level. That is, they prayed God would not permit the perpetrators of evil deeds to continue to govern the righteous. Judgment of the act, i.e. of the sin, was called for.
White robes were given them (Revelation 6:11), which signified the righteousness of the saints (Revelation 19:8; Isaiah 61:10; Psalm 51:2). In other words, it wasn’t a righteousness of their own, but that acquired through confession of their sins and coming under the blood of Christ (cf. Revelation 7:9, 13-14).
I don’t believe these martyrs at the base of the altar represent the saints of the Old Covenant, as is normally assumed, because, if that were true, the reason for their martyrdom would contain a redundancy. Notice that they were slain “for the Word of God **and** the testimony, which they held (Revelation 6:9). If the Word of God means Scripture, what does the testimony they held refer to? Wouldn’t their testimony also refer to Scripture, whether under the Old Covenant or the New (cp. Revelation 12:17)? The “testimony of Jesus Christ” represents those Scriptures that proved Jesus is the Messiah (Acts 17:2; 18:28; 1Corinthians 15:3-4; cp. Luke 24:27)? Therefore, the Word (Logos – G3056) of God must refer to Jesus, and the souls must refer to slain believers under the New Covenant. In other words, they were slain for Jesus’ sake and for their testimony about him.
Earlier, the Scriptures show that overcomers walked with Jesus in white raiment (Revelation 3:4-5), and those who did depend upon their own righteousness were encouraged to buy white robes from the Lord, so that their nakedness could be hidden (Revelation 3:18). The righteous were told to rest for a little while, until their fellow servants and brethren were killed, as they were, should be fulfilled (Revelation 6:11). A fellow servant is one who has the same master as the first servant, and they were told that their vindication was delayed only a short time. It was desirable for all to vindicated together in the presence of their enemies.