Revelation 7:9, 13-14

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

Then one of the elders addressed me, saying, “Who are these, clothed in white robes, and from where have they come?” I said to him, “Sir, you know.” And he said to me, “These are the ones coming out of the great tribulation. They have washed their robes and made them white in the blood of the Lamb.