Revelation 7:9, 14

After this I saw a vast crowd, too great to count, from every nation and tribe and people and language, standing in front of the throne and before the Lamb. They were clothed in white robes and held palm branches in their hands.

And I said to him, “Sir, you are the one who knows.” Then he said to me, “These are the ones who died in the great tribulation. They have washed their robes in the blood of the Lamb and made them white.