The Separation of God's Folk
"Come out from among them and be separate."
"You are ... a separate nation."
Paul of Tarsus, 2nd Corinthians 6:17; 1st Peter 2:9. (Both authors are quoting from the Old Covenant Book but applying it to life under the New. Throughout the New Covenant Book hagios, usually rendered in European languages by words like "holy", really means "separate".)
Healthy human development is when life develops in accord with natural principles, and is the way to prepare for union with God. This has always been humanity's task, but the old humanity shirked it and only the new humanity can do it.
The old humanity are not only incapable of leading healthy development: they spoil any development they are part of, so reborn souls need to come together as a separate nation, acting together in every realm of activity and guarding against infiltration by unreborn souls. The new life prompts every reborn soul towards this, however circumstances may hinder.
"Only God knows who is reborn so we should not judge."
It is true that only God knows for sure, but we can make assessments, cautiously and provisionally but firmly, based on their professed creed and practised deed. And we need to make such assessments, for otherwise the whole idea of separation is empty. The New Covenant Book contains several examples of this. Matt 18:15-17; 1 Cor 5:1-13; 2 Cor 6:17; 1 Thess 5:14; 2 Thess 3:15; 1 Tim 5:20; Heb 12:5-12; 2 John 1:10.
And all this still applies if we are assessing Christians. Simply associating with Christian traditions or groups is no guarrantee of having been reborn. When Christianity became popular there were strong selfish motives to join the movement, especially after dominant regimes jumped on the bandwaggon. If the Christian professes the kind of creed we would expect of a reborn self, that is a good sign, but professed creed needs to be matched by practised deed.
There may be more reborn selves, Christian or otherwise, than it seems, misguided rather than obstinate. We have no way to distinguish; we can only judge by the path they take. And if the path is the broad path, we are bound to assume that they are not reborn unless and until they bear fruit that shows otherwise.
"Such Isolationism would prevent reborns bringing the good news to the world."
No it would not. Surrounding nations can observe. International conversation can occur. Evangelists can go into all the world.
The other way round is more likely, for being involved in the world makes the new life seem like just another leisure activity. "Bill likes to talk about Football and Ben likes to talk about Jesus." Thus, unreborns hear more but listen less.