A surface reading of Revelation chapter seven suggests that the two groups are totally different. The 144,000 is a specific number of Jews made up of a specific number from each of the twelve tribes (7:4-8). The Great Multitude, on the other hand, is an innumerable collection of Gentiles from every nation, tribe, people and language (7:9). The 144,000 is also called “first fruits” in 14:4, implying that there is a second group like them in some way. But closer reading of these texts militates against those initial impressions.
First of all, the terms used for God’s end-time people are often interchangeable in Revelation. God’s people are not only called 144,000 and great multitude, they are also called remnant (12:17), saints (14:12), those who keep their garments (16:15) and the called, chosen and faithful followers of the Lamb (17:14). And several of these names are explicitly interchangeable. Two examples. 1) God’s people are called “remnant” in 12:17, then 144,000 in 14:1. But 14:1 alludes to Joel 2:32, where the same group is called “remnant.” The two groups have different names in Revelation but are the same end-time group. 2) The 144,000 of chapter 14 are then called “saints” in 14:12. So remnant, 144,000 and the saints are different ways of describing the same end-time group.
Second, John never sees the 144,000, he hears the number (7:4). But after hearing the description of the 144,000 (7:4-8) he looks and sees an extremely large group that no one can number (7:9). This hearing/seeing comparison is a literary pattern throughout the book of Revelation. John hears one thing (Lion) then sees its opposite (Lamb), but the two are different ways of describing one reality (Rev. 5:5-6). He hears a voice like a trumpet, but when he looks he sees the son of man speaking to him (Rev. 1:10-12). John hears that a prostitute is sitting on many waters, but when he looks he sees a woman sitting on a scarlet beast (17:1, 3). In each case, the two images are in strong contrast, even at opposite poles (like lion and lamb), yet they are different images that described the same thing.
Third, in Revelation 14 there are two harvests, the wheat and the grapes. So when the texts speaks about the 144,000 as first fruits (Rev., 14:4), the “second fruits” are explicitly mentioned later on in the chapter. The wheat grains, representing the righteous, are the first fruits of that harvest. The grapes, on the other hand, are the second fruits or the completion of the harvest image. The 144,000 in Revelation 14, then, are not a separate group or a portion of the whole, they themselves represent the entirety of God’s end-time people.