Question: I have a question about Isaiah 6:10-13 where the remnant of believers is said to be a "tenth". Does this refer to those saved on judgment day? How many will be in the 10% remnant? How many Christians are there anyway, and how many will be saved through the Tribulation?
Response: The question is, 10% of what? Isaiah 6:13 is 1) talking about the remnant of Israel which survives the coming Tribulation; and 2) suggests that even that "tenth" will be further pared down: "For although a tenth remains in it (i.e., the land of Israel), it will again be for burning (i.e., will still be subject to refinement)". The meaning of this verse is that the future crisis of the invasion of antichrist (which is foreshadowed by the contemporary invasion of Assyria described throughout the book of Isaiah) will reduce the population of Israel drastically. And even though as many as a tenth may survive, that number will still be larger than the true remnant. That is, they will still be refined by our Lord when He returns. This refining of the remnant of Israel is a subject treated in many portions of the Old Testament, and the prophecy of Israel's regathering followed by a purging that will take place at our Lord's hands occurs very frequently (e.g., Is.1:25-31; 4:4; 6:13; 10:20-22; 11:10-16; 35:8-10; 43; 49:8-26; 59:20-22; 60:4; 65:8-12; 66:20; Jer.3:18; 30:10; 31:8; 31:30-34; 33:8; 33:15; Ezek.11:17; 20:33-38; 20:41; 34:13; 36:24-38; 37:11-14; 37:21-28; 39:25-29; Hos.1:10-11; 6:11; 11:9-11; Amos 9:9-11; Zeph.3:9-13; 3:20; Zech.8:7-8; 10:10; 13:1; Mal.3:3; 4:5-6; cf. Matt.23:39; Rom.11:25-27).
So Isaiah 6:13 does not really tell us about believers at all, but rather about that percentage of the unbelieving Jewish population who will survive the Tribulation to come into judgment in the wilderness before entering the land. These will form the nucleus of the Messiah's millennial kingdom, while believing Jews will be resurrected along with gentile believers at the return of our Lord at His Second Advent (see Peter lesson #20: the Resurrection). Finally, since this "one tenth" is an estimate of what may well be a lower amount - "even if as much as a tenth remains" - and since it is later reduced by refinement and purging, the number who actually enter Christ's kingdom may be relatively small indeed.
We do not know how many believers there are in the world, nor how many will survive the Tribulation. We do know that some will, for those who do are resurrected bodily at our Lord's return (1Thes.4:13-18; cf. 1Cor.15:23). We also know that very many genuine believers will be martyred during that last seven years for their faith in Jesus (in the Great Persecution; cf. Rev.7:9-17). Finally, we also know that a large number of so-called and lukewarm Christians will fall away from the faith during the Tribulation in the Great Apostasy (covered in part 3A of the Coming Tribulation series). Only God knows the answer to the question of how many Christians there are in the world. We are told to be innocent as doves and wise as serpents (Matt.10:16; cf. Rom.16:19; 1Cor.14:20). That is, we should not assume that everyone who claims to be a follower of Jesus really is so just because they say so. On the other hand, we should be careful about making this judgment ourselves based upon our own ideas of what constitutes saving faith. Since we cannot know for sure who is and who isn't, and cannot come up with a number even now, it is impossible to tell, in the absence of any statement in the Bible, how the number will work out. But it is clear from scripture that during the Tribulation only a small percentage of mankind will survive (Is.13:12). Given this fact, and the Great Apostasy and the Great Persecution, we can expect the number to be smaller rather than larger.
Please also see the link: The Regathering and Purging of Israel
Hope this helps!
Yours in Jesus,