Ethno-nationalism is the only political system specifically endorsed in the Bible.  If Christians are forbidden to advocate ethno-nationalism they are forbidden to advocate any political system whatsoever. Stated differently: If ethno-nationalism is un-Christian than so are all other political organizations because God has never presented any other program. 
Stated differently: God never advanced a political program contrary to his Old Testament establishment of ethno-nationalism.
Stated differently: If Christians are barred from advocating ethno-nationalism we are barred from all political advocacy whatsoever. The only political ideology God ever encouraged was ethno-nationalism.
The ethno-nationalist state of Israel was the finest and most just polity in history because its legislator was God. 
The concept of nation divorced from ethnicity has no Biblical precedent. Nation equals ethnicity. “Propositional nation” is unbiblical.
The Apostle Paul said: “All Scripture is breathed out by God and profitable for teaching, for reproof, for correction, and for training in righteousness.”  He was speaking of the Old Testament,  a document which clearly advocates ethno-nationalism. The burden of proof, then, lies on the opponents of ethno-nationalism to find its repudiation in the New Covenant. 
(1) If God allows Christians to encourage democracy, which he never endorsed, why would he condemn Christians for encouraging ethno-nationalism, which he has endorsed?
(2) Does the New Testament repudiate the only political system God ever established while allowing for most anything else? 
(3) Did God establish an evil Old Testament system just to make a statement by repudiating it in the New?
(4) How could (3) be reconciled with Old Testament verses claiming ethno-nationalist Israel was to be a standard for gentile nations ?
 Ethno-Nationalism: A form of nationalism wherein the “nation” is defined in terms of ethnicity. Ethno-nationalists believe nations are defined by a common heritage, a common language (typically), a common faith, and a common genetic ancestry.
 The Kingdom of Israel was an ethno-national state established by God for the genetic descendants (“seed”) of Abraham.
Deuteronomy 17:15: “You may indeed set a king over you whom the Lord your God will choose. One from among your brothers you shall set as king over you. You may not put a foreigner over you, who is not your brother.”
 A consistent (and more respectable) case might be made that Christians must be a-political. I am not addressing this argument.
 The New Testament never presents a political program to replace Old Testament ethno-nationalism.
 Riley, Patrick: “Chronology in Jacque-Benigne Bossuet, Politics Drawn from the Very Words of Scripture” lxix, lxix-lxx (Patrick Riley trans. and ed., Cambridge Univ. Press 1990) (1704). Via: [Gee, Todd. “Monarchist and Democratic Christian Perspectives Preceding and Subsequent to the Reformation: A Survey of Selected Authors,” 2015.]
 II Timothy 3:16
 The New Testament did not exist at the time of Paul’s writing.
 These opponents believe in massive discontinuity between the Old and New Testaments.
 Post-Constantine, Christians embraced empire. A millennia ago, almost all Christians were monarchists. A century ago, Christians were imperial colonialists. Today, Christians overwhelmingly support democracy and ethnic pluralism.
 Deuteronomy 4:5-7: “See, I have taught you statutes and rules, as the Lord my God commanded me, that you should do them in the land that you are entering to take possession of it. Keep them and do them, for that will be your wisdom and your understanding in the sight of the peoples, who, when they hear all these statutes, will say, ‘Surely this great nation is a wise and understanding people.’”