They have washed their robes white again in the Blood of the Lamb
Revelation 7:13–15 (NJB): One of the elders then spoke and asked me, ‘Who are these people, dressed in white robes, and where have they come from?’ 14 I answered him, ‘You can tell me, sir.’ Then he said, ‘These are the people who have been through the great trial; they have washed their robes white again in the blood of the Lamb. 15 That is why they are standing in front of God’s throne and serving him day and night in his sanctuary; and the One who sits on the throne will spread his tent over them.
During the Judgement time in vision to St John, an elder asked him who is those worn the white gowns. They are people who have martyred for the glory of God. They have shed their blood like their Master has shed his Blood. They have experienced great trial from rulers of this world and they resist and never compromised and stand strong to witness Jesus. They imitated Christ in their life and never afraid to give their life to Christ Jesus. In Heaven they have the place in front of God’s throne for serve him day and night. God have given the right place for them to be with Him under His protection and giving them His tent to live in His presence.