They are all “clothed in white robes.” The martyred souls under the altar had also been given white robes to wear (Revelation 6:11), but now John sees many more dressed in white. This multitude has received forgiveness of sins through the blood of the Lamb (see Revelation 7:14). Those washed in Jesus’ blood are also renewed by his Holy Spirit (Romans 8:9). This explains how a crowd divided by language and culture and customs so as to be distrustful of one another can be together peacefully (Ephesians 2:11–22).
9 After this I looked, and behold, a great multitude that no one could number, from every nation, from all tribes and peoples and languages, standing before the throne and before the Lamb, clothed in white robes, with palm branches in their hands,