Michael Wilcock answers in his commentary on Revelation:
The sealing: who? (verses 4–12)
There has been much dispute over these verses. Who are the 144,000 of verse 4, who are the innumerable crowd of verse 9, and what is the relation between them? But it is less of a puzzle than might first appear. Verse 3 has told us who are sealed: it is God’s servants. We have no reason to limit this in any way. The servants of God, all of them, Old Testament and New, all his believing people, are sealed. If we are his servants, the message is for us as much as for any (1:1), and we too are sealed.
But if this is so, how can the sealed be described in verse 4 as 144,000 Israelites? Many theories, as wild as they are unnecessary, have been built on this description. The plain fact is that if we are God’s servants, we have been sealed. If we are then told that we number 144,000, when we know very well that there are millions of us, the figure is presumably another of the symbolic figures of Revelation, and indeed it looks too stylized to be anything else—the suspiciously tidy sort of number that is much more likely to be a symbol than a statistic. If we next find ourselves described as 144,000 Israelites, when most of us are Gentiles, this is in line with the regular New Testament teaching which applies to the Christian church the titles and privileges of Israel, and which we have already noted in 3:9. If our numbers are specified even more closely, and oddly, in that each of the twelve tribes, whether large or small, contributes just 12,000; and if the tribes are listed in an order found nowhere else in the Bible; and if one of them (Dan) is omitted altogether, and the lack made up by including one of Joseph’s sons as well as Joseph himself; then the description of us is very stylized indeed. But it is the kind of description we should expect if this is a ‘diagram’ of the church. Like every diagram, it sacrifices one kind of accuracy to clarify another; as when a map projection will sacrifice true distance in order to depict true area, or vice versa.
Then what about the innumerable crowd of verse 9? What is the relation between them and the 144,000? They are one and the same. For whatever else the white-robed multitude may be, they are certainly servants of God; and if they are servants, they are sealed (verse 3); and if they are sealed, they are the 144,000 (verse 4). But how can this be—how can a limited number, all Israelites, be the same as a numberless multitude drawn from every nation? Yet again, we put ourselves in John’s place. What he heard was a voice from heaven, declaring the results of God’s census of his people. More than once in Old Testament times, and again, significantly, at the coming of Christ (Lk. 2:1–7), they were caused to ‘stand up and be counted’; and here is God’s own count of them. The total may be a symbolic number, but it is still a number. If God can count the very hairs of our head (Mt. 10:30), a counting of the heads themselves is unlikely to be beyond him! ‘The Lord knows those who are his’ (2 Tim. 2:19), and what John heard was God’s declaration of their total, given symbolically as ‘144,000’. What he saw, on the other hand, was that this definite total, known to God, is from the human point of view a numberless multitude. Similarly, from God’s standpoint they are all ‘Israel’, his people; from our standpoint, they come from every nation under heaven.*
*Wilcock, M. (1986). The message of Revelation: I saw heaven opened. The Bible Speaks Today (79–81). Leicester, England; Downers Grove, IL: InterVarsity Press.