who have come out of: See Revelation 6:9; 20:4.
the great tribulation: A period of terrible and distressing events (see Revelation 1:9; 2:9–10; Deuteronomy 4:30–31; Matthew 24:21–22; Acts 14:22).
they have washed their robes … in the blood of the Lamb: This description is a symbol of Christ’s redeeming death for them.
made them white: This phrase shows their victory over sin and death and their acceptance by God into eternal life.