Those are some accurate non-universalist interpretations (which also work for annihilationism, as I’ll show in a minute). And yes, it’s not a good idea to dump this or that non-universalist eschatology simply because the idea sounds intolerable. Although, my Teacher C. S. Lewis (in the sense that he honored MacDonald as his own Teacher, so I’m following in their rabbinic school, so to speak ) used to insist that “hell is intolerable” while talking about how people in hell choose to tolerate it after all, so that he could tolerate it as a doctrine after all. So there’s intolerable, and there’s intolerable, apparently.
Don’t let anyone on any side of the aisle (of any aisle!) tell you the answer is easy and simple to get to. We’re talking about the Bible (and about metaphysical logic). It is freakishly detailed with lots of different kinds of details. No one should quickly come to a conclusion about what it’s teaching or not teaching in any direction.
So, keeping that in mind, and thus also in mind that trying to figure out the meaning of highly poetic dream imagery is not necessarily going to be easy, even by other Biblical standards : what, if anything, can be systematically analyzed about that passage?
1.) Are they entering into the city or not? The translation you gave is accurate to the language in Greek: they’re entering into the city where no one still fondling their sins can enter, such as those who are still practice idolatry and dishonesty – not unless their names are written in the Lamb’s Book of Life.
The Greek at the end there however is a bit unusual; it’s more like a predictive condition. In English it would be more accurately translated something like, “not until their names are written”. This has some bearing on what’s happening toward the end of Rev 22 – since those currently outside the city are being evangelized to repent of their sins and come in, and they’re being at least compared to those in the lake of fire outside the city who are still fondling their sins. This is so very obvious that the only way not to get post-mortem salvation out of it is to argue that the evangelical call is not in the same ‘scene’ as those still outside the city after the descent of the NJ (but the Bride is the NJ, and the exhortation is clearly to come into the NJ, so it’s hard to suddenly dichotomize the call as though it cannot apply to after the judgment but only before the judgment); or to argue that the call simply always fails or is only some kind of useful legal fiction to go through the motions because that’s what good people do. I used to hear both kinds of interpretation when growing up (RevJohn interpretation being very popular in Southern Baptist churches since the late 70s, to say the least); it only occurred to me much later that they cancel each other out very neatly, and that the evidence each theory acknowledges points to post-mortem evangelism and salvation.
Anyway. Whether the Greek at the end of 21:27 is a predictive condition, also has some connection to who those kings of the earth are entering into the city, following the light of Christ. But they aren’t only coming up to the gates and throwing something in (their glory? their honor? the nations who are following them?), and then plopping forever outside or being annihilated afterward. They’re entering the city.
2.) Are the kings of the nations saved or not? Relatedly, are they are thus being saved or not? Well, yes, if they’re following the light of Christ and entering where no one can enter unless/until their names have been entered in the Lamb’s Book of Life, then they’re dang well saved. This is so obvious that you’re the first person I’ve ever heard of who considered a theory that they were only going up to the gates but not going in, only sending in their… whatever. (Not that I’m complaining; I very much admire the creative option of the attempt!) But they’d have to be sending in their own honor and glory, and that in itself would be (by Ancient Near Middle Eastern standards, and Mediterranean standards more broadly) an action of true loyalty declaration, like casting down one’s own crown at the feet of the high king or emperor (to borrow a not-incidentally connected image from earlier in RevJohn!) Ditto for the kings sending in the nations who were following them to the City, too. Ditto for the nations bringing their own honor and glory into the NJ.
3.) What was the relevant former status, if any, of those kings of the nations? That phrasing is only ever used in RevJohn elsewhere to talk about the worst human rebel leaders (under the Unholy Trinity, if any of them are human, as I expect at least two of the three are supposed to be.) The last time we saw them, before the descent of the NJ, they had been slain by the shepherding iron rod of Christ (using imagery that fits surprisingly well with the end of Psalm 23 by the way!) Who had scattered their bodies for the birds to feed on.
If even they are coming in, that not only counts as testimony for post-mortem salvation, probably even out of the Lake of Fire (since it’s hard to imagine they of all people didn’t go into the LoF judgment along with their unholy masters, and John says earlier that even mere minions wearing the mark, whatever that actually means, go into the LoF judgment, so again kings slain in their ultimate rebellion would be slated for that); it also points toward a greater-includes-the-lesser comparison. If even they are coming in eventually, that points to the totality of lesser sinners coming in, too, such as the nations who followed them in their rebellions.
And the prophetic OT imagery being drawn on here, confirms that rebel kings who once prosecuted God’s righteous people, are being reconciled with them and back into loyalty to God. Back in the OT imagery, that might only involve those rebel kings who happened to survive the coming of YHWH visibly (as in a millennial reign theory before the general resurrection and final judgment – for which I think there is quite a bit of testimony even in the OT, by the way). RevJohn makes it abundantly clear that rebel kings should not be expected to survive the coming of YHWH visibly, nor the zorching to come at the end of the visible millennial reign (if RevJohn 20 is talking about that).
Now, all that put together, so far as it goes, could still be short of universal salvation in a couple of ways. Maybe the greater-includes-the-lesser comparison is only a general expectation to which there will still be exceptions, some people who don’t follow the kings into the New Jerusalem after all – or those people have been annihilated already, and everyone not annihilated is coming in. Or, along a line more like what Iraeneus seemed to be teaching, maybe all humans will be saved even post-mortem (and post LoF judgment) but the rebel angels won’t be, being annihilated instead perhaps.
But that’s one way a Christian universalist could reply to Rev 21 and to non-universalist critiques of it. Yes, it’s the kings of the earth who were previously slain and sent into the LoF, and they aren’t impenitent dishonest idolaters anymore. No, it is the same “kings of the earth”; the phraseology (in an early RevJohn chapter) about some or all Christians being kings and priests is quite different and has a different topical thrust. Yes, of course following the light of Christ into the NJ and bringing one’s own honor and glory into the city, is a picture of them being saved, just like slaking one’s thirst and washing one’s dirty robes in the river of life which flows out of the never-closed gates. If even those kings are going in, leading the people who follow them, then we should expect post-mortem evangelization to be widely successful. No, that doesn’t mean that anyone can just traipse into the New Jerusalem (allegorically speaking or otherwise) on their own terms whenever they want: people who continue to impenitently fondle their sins and other idolatrous rebellions must remain outside, so long as they continue to do that. But they can obviously repent and come in, too; even if the evangelical activity of the Bride (the NJ itself) and the Spirit are discounted later, the light of Christ is leading them in, including the rebel kings who had themselves been slain by Christ as rebels.