Rev. 7:13 Then one of the elders asked me, “These in white robes—who are they, and where did they come from?” 14 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.
Revelation 7:9 - 17
ESV - 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. 10 And crying out with a loud voice, "Salvation belongs to our God who sits on the throne, and to the Lamb!
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