ISB ~ Revelation 7 v14

great tribulation:A time of unprecedented distress triggered by the opening of the seals (6:1-17; Dan 12:1). Some link this with the “great tribulation” that Jesus warned would engulf the Roman world in connection with the violent conquest of Jerusalem (Mt 24:21); others link it with the Domitianic persecution of Christians near the end of the first century. See note on Rev 3:10.

they have washed:The blood of Christ whitens the robes of the saints (6:11).

• The rite of priestly ordination in Israel included the purification of priestly garments with blood (Lev 8:30). The sacrificial blood of Jesus likewise consecrates believers for service in the heavenly temple (5:9-10; 7:15).