Who are the people that enter the Kingdom and are able to reproduce, creating the unbelievers of that time? Are they Gentiles or Jews?
Taking all that we have in Scripture together from our Revelation and Isaiah studies, we come to the conclusion that there are believing Gentiles who live to the end of the Tribulation (i.e., the "sheep"), who then enter the Kingdom as natural men. As natural men, they can marry and reproduce just as natural men do today, and their children may or may not become believers themselves just as happens today.
This group of people is responsible for repopulating the Kingdom, and they are the source for the unbelieving armies of men who rise up against Christ at the end of the Kingdom when Satan is let loose (Rev 20:7-8).
We also know that at the end of Tribulation, a separate group of orthodox Jews is brought to faith in Christ and form the remnant of Israel who is saved on the last day (i.e., Zech 12:10; Rom 11:25-27; Isa 59:20-60:4). We also know that in the Kingdom, the Jewish nation is described as consisting entirely of believers (i.e., Jer 31:34). So, the more difficult question is whether the Jewish nation also has natural men in it, who may reproduce and have children?
Looking at one of the passages quoted above (Isa 59:20-21) and in Isaiah 66:18-23, we find references to descendants or offspring, leaving us to wonder whether the Jewish remnant that comes out of the Tribulation will also enter the Kingdom in natural bodies. In our Revelation teaching (recorded in 2005), Pastor Armstrong taught that the Jewish remnant that enters the Kingdom will receive new bodies during the 75-day interval, based on passages like that found in Isaiah 25 that seem to suggest the nation of Israel is entirely regenerated and not given to marriage and reproduction.
In recent years his teaching has stepped back from a firm commitment to this position, especially in light of passages in Isaiah 66 and elsewhere that suggest Jewish reproduction. Dr. Arnold Fruchtenbaum of Ariel Ministries believes that the Jewish remnant that enters the Kingdom from Tribulation is like the Gentile believers: they are natural men, and therefore they will reproduce. Unlike the Gentile believers, however, Fruchtenbaum believes that based on the promises found in Jeremiah 31:34 and elsewhere, every Jewish child is assured to come to faith during their lifetime, since God promised the nation they would have no unbelievers in the Kingdom.
Obviously, this is a difficult area of Old Testament prophecy, and depending on how you view the various texts in Isaiah, you may believe that either all Jewish saints are resurrected into a new, incorruptible form (and therefore can't reproduce), OR you believe that some Jews will enter the Kingdom in natural form and can reproduce and then later their Jewish children become believers by the power of the Spirit. Our ministry is unsure of which view is intended by Scripture, though we teach the former view as the more likely interpretation.