Then one of the elders asked me, “These in white robes—who are they, and where did they come from?”
14 I answered, “Sir, you know.”
And he said, “These are they who have come out of the great tribulation; they have washed their robes and made them white in the blood of the Lamb. (Revelation 7:13-14)
When John sees the great multitude of people from all ethnicities (7:9), he naturally wonders who they are. One of the elders before the throne tell him clearly that these are those “who have come out of the great tribulation.” It would seem that these are not people who escaped the tribulation, but people who suffered through it and died. Most of them were probably executed for their faith (Revelation 6:9). When it says that “they have washed their robes and made them white in the blood of the lamb” it means that they are true believers whose sins have been forgiven on the basis of Christ’s sacrifice on the cross. It is a fitting and beautiful thing that they were permitted to have their own blood shed for the one who shed His blood for them.
On final note: this passage implies that the persecution of believers during the tribulation period will happen all around the world, since people of every nationality will be among the martyrs. This also indicates that the gospel will be successful in penetrating the farthest reaches of the Earth.