Thematic Bible: No entrance into heaven without cleansing

Then one of the elders answered, saying to me, "These who are clothed in the white robes, who are they, and where have they come from?" I said to him, "My lord, you know." And he said to me, "These are the ones who come out of the great tribulation, and they have washed their robes and made them white in the blood of the Lamb.