Following the opening of the six seals, John remains in the throne room where he sees 144,000 people, 12,000 from each of the various tribes of Israel, who have been sealed as servants of God.
Beyond this, suddenly, he also then sees a multitude of people from each of the other groups of people across the world, all in white robes. An elder from that throne room explains to John that these are people that have come from the great tribulation.
The Lamb, Jesus himself, in Revelation 6, has just opened six different seals which unleashed this tribulation upon the earth, the wrath of God that came as a wave upon another wave. Now, those who have come from that time have arrived in the throne room in these white robes:
Then one of the elders asked me, “These in white robes —who are they, and where did they come from?”
I answered, “Sir, you know.”
And he said, “These are they who have come out of the great tribulation; they have washed their robes and made them white in the blood of the Lamb.
Revelation 7:13-14
It is a type of paradox that, having washed their robes in blood, they could turn out white. Blood on clothes would stain them red, but yet in the blood of the Lamb, it is made white.
Those that are standing in the throne room praising God, serving him night and day, sheltering in his presence and being led by the Lamb, these people are the ones that have washed their clothes, making them white, in the blood of the Lamb.
We can do the same. We can be cleansed. We can be made clean and white as we stand before God by placing our faith in Jesus. We can come to Christ with our dirty garments, those that have been made dirty by our disobedience, our rebellion against God, our sin, and we can wash them in the blood of Christ. By placing our faith in his sacrifice, we can be cleansed of our sins and be reconciled with God. If we want to be among those who are able to stand in God’s presence one day, we can be washed, making our clothes white again, in the blood of the Lamb.