We may be inclined to say that salvation is redemption from sin and so amounts to receiving forgiveness of sins (with its blessed consequence of life eternal). That understanding is accurate but not complete, and certainly does not help us get to the heart of why this great multitude speaks “with a loud voice.”
We need to remember the context, which is one of fright, panic, and desperation, as described in Revelation 6:17. The apostle Paul says in Romans 1:18 that “the wrath of God is revealed from heaven against all ungodliness and unrighteousness of men, who…suppress the truth.” The plagues unleashed with the opening of the seals of Revelation 6:1–17 illustrate and exemplify that wrath. The last of the plagues thus far (Revelation 6:12–17) had prompted everyone, slave and free, to utter the desperate cry to the mountains to fall on them so as to hide them from the wrath of him who sits on the throne and the wrath of the Lamb (Revelation 6:16). Such a cry came from the conviction that no one could stand in the face of that wrath.
But here is now the marvel: this great multitude can stand, and can even stand “before the throne and before the Lamb” (Revelation 7:9). That is the salvation they delight in with that single voice. Yes, they are dressed in white robes signifying forgiveness of their sins, and that is so wonderful and exciting because it means they are spared the infinite and terrible wrath of God.
10 and crying out with a loud voice, “Salvation belongs to our God who sits on the throne, and to the Lamb!”