Revelation 7:
9After this I looked and saw a multitude too large to count, from every nation and tribe and people and tongue, standing before the throne and before the Lamb. They were wearing white robes and holding palm branches in their hands.
10And they cried out in a loud voice:
“Salvation to our God,
who sits on the throne,
and to the Lamb!”
11And all the angels stood around the throne and around the elders and the four living creatures. And they fell facedown before the throne and worshiped God,
12saying, “Amen! Blessing and glory and wisdom and thanks and honor and power and strength be to our God forever and ever! Amen.”
13Then one of the elders addressed me: “These in white robes,” he asked, “who are they, and where have they come from?”
14“Sir,” I answered, “you know.”
So he replied, “These are the ones who have come out of the great tribulation; they have washed their robes and made them white in the blood of the Lamb.