The Great Multitude Of Many Nations, as mentioned in Revelation 7:9-17, refers to the great "soul harvest" of people from all corners of the world who become believers through the preaching of the 144,000 sent out into all the world. This takes place from Soul Harvest until Glorious Appearing. Revelation 7:9 says that this is a great multitude that no one can count from every nation, land, tribe, and tongue. The authors may have interpreted Revelation 7:16 as indicating that God has conferred immunity from the blood plagues and heat of the Bowl Judgments to the Great Multitude.
In Apollyon, Tsion Ben-Judah stated that it thrills his heart that there are likely more Christ-followers during the Tribulation than were the number of people who had been raptured. He said that even nations with a minuscule number of Christians in the past now have many people coming into salvation.