A328 : by Tony Garland 
I believe that Christ will (ultimately) be judging all mankind—although His function as ruler/judge will over a perfect government made up of glorified saints who co-reign with Him, from both believing Gentiles and Jews as I describe in my Revelation Commentary sections titled Millennial Reign of Messiaha, Millennial Reign of the Saintsb, and Judgment by the Saintsc.
I do not take "nations" to indicate a separate class being judged than individuals. Nations are made up of individuals. The term nations emphasizes all the different ethnic groups which are comprised of individuals.
It is my view that, following the second advent, the earth is cleansed of all unbelievers such that only believers enter the millennial kingdom (Mat. 13:30, 41, 49-50; 24:40-41; Rev. 14:15,19). The kingdom of God arrives on earth at the second advent and begins with the millennial kingdom. No one lacking the righteousness found in Christ by faith will be allowed to participate. Among other things, I believe Jesus makes this clear in the parable of the wedding feast (Matthew 22:11-12 cf. Isa. 61:10).
Numerous passages indicate that true faith is demonstrated by action (Mat. 25:34-40; Jas. 2:17-18; Rev. 19:8). But these passages must not be misconstrued as upholding works-based salvation (Rom. 4:5-6).
Also: the idea that Jesus would say the following to unbelievers seems incredible to me, "Come, you blessed of My Father, inherit the kingdom prepared for you from the foundation of the world..." How can we say that unbelievers would be called blessed or have an inheritance in the kingdom of God—which would have to be the case if unbelievers find entry.
It is important to make a distinction between "nations," "sheep," and "goats." Nations are groups of people made up of both sheep and goats—representing individuals. There are not "sheep nations" and "goat nations." It is not nations which find entry into the kingdom, but individual sheep. Similarly, no nation in toto finds itself cast into the lake of fire, but individual goats from among the nations.1
I do not believe the passage teaches that there are "sheep nations," as you put it. There are 1) nations; 2) sheep [from among the nations]; 3) goats [from among the nations]; and Jesus' brothers [Jews, all of whom will be believers by this time]. So the primary divisions are 1) nations vs. Jews; 2) then from among the nations: sheep (believing individuals) vs. goats (unbelieving individuals).
Also, if unbelievers were to be admitted into the millennial kingdom it would undermine one of the purposes for the millennial kingdom: a historic dispensation which begins with ideal conditions: Satan bound, a perfect government made up of Jesus and glorified saints, and a 100% believing unglorified population—but which goes astray nonetheless and produces a brood of unbelieving rebels (Rev. 20:7-9). See Why a Millennial Kingdomd?
I hope that helps. I think your main confusion stems from mis-identifying sheep and goats as nations rather than individuals.
Endnotes:
| 1. | This is one of the sound arguments against the view that the beast and false prophet represent institutions—since both of them are individually cast into the Lake of Fire (Rev. 19:20). |
| 
