Revelation 7:9, 13-14

After this I looked, and there before me was a great multitude that no one could count, from every nation, tribe, people and language, standing before the throne and before the Lamb. They were wearing white robes and were holding palm branches in their hands.

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.