In Revelation 6:9-11 "the souls of those who had been slain for the word of God and for the testimony which they held" are crying out with a loud voice, saying, "How long, O Lord, holy and true, until You judge and avenge our blood on those who dwell on the earth?"
In Revelation 7:9-17 these same souls are no longer "under the altar" (Revelation 6:9). Now they are "before the throne and before the Lamb" (Revelation 7:9). They are still crying out with a loud voice mind you. Here what they are saying now: "Salvation belongs to our God who sits on the throne, and to the Lamb!" (Revelation 7:10).
Before they were in "great tribulation" (Revelation 7:14). Now "they are before the throne of God," serving Him, dwelling with Him (Revelation 7:15). They no longer hunger or thirst, suffer, or cry tears (Revelation 7:16-17). The Lamb shepherds them and leads them "to living fountains of waters" (Revelation 7:17).
From under the altar to before the throne because of the Lamb. The Lamb delivered these souls. He will deliver yours too.
Jason Cicero