The great multitude of Revelation 7:9, 13-14 are those who have come through the Tribulation soon to afflict the earth. This group consists of people who were never in God’s true Church prior to the Tribulation.
Right after this frightful time, when men are seized with stark terror by supernatural signs in the sun, moon and stars, multiplied thousands will repent. When they see how the world was misled and deceived about the great Tribulation, and they have gone through it, then at last so many that they cannot be numbered (figuratively – Rev. 7:9), will recognize the truth and turn to God, rather than the superstitious nonsense of denominational churches and pagan religions of this world. They come out of the Tribulation and wash their robes, making them white in the blood of the Lamb, meaning they become converted and accept the blood of Jesus Christ to pay for their sins (Rev. 1:5).