Revelation 7:13-14

13Then one of the elders responded, saying to me, “These who are dressed in the long white robes—who are they, and from where did they come?”

14I said to him, “My lord, you know [the answer].” And he said to me, “These are the people who come out of the great tribulation (persecution), and they have washed their robes and made them white in the blood of the Lamb [because of His atoning sacrifice].