I'm only going to speak personally, but I'd like to think my views have been informed both biblically, by the Lutheran Confessions, and also by the ancient fathers.
I don't see much room for overt nationalism in my life as a Lutheran. I think patriotism, a love of one's country and a desire to see one's neighbors prosper, is in line with Lutheranism and Christianity more broadly. But overt nationalism muddies things, it creates a fogginess, it creates or at least encourages forms of theologia gloriae, theologies of glory.
A significant influence in my thinking here is also rooted in some of the writings of, and experiences of Pastor Bonhoeffer and the Confessing Church in the Kirchenkampf, the "Church Struggle" that arose as a product of the Nazi regime and the hostile take-over by the Deutsch Christen (the pro-Nazi movement within the German Evangelical Church). A product of this struggle was the Barmen Declaration, which while authored by the Reformed theologian Karl Barth was nevertheless a significant work of Christian confessionalism in a time of intense struggle and difficulty.
Of particular importance here is where the Barmen Declaration asserts that the Church can only have one Lord, and must therefore reject all others that would seek to take that position from Him, that the entire Christian life is subsumed into Christ and the Christian finds his/her full duty in Christ; thus other "Leaders" have no place. This is not a rejection of the Office of the Keys (heaven forbid!) rather it is a rejection of an overtly-nationalistic and State occupation over the Church seeking to impose its will over the Church and claiming Christians owe allegiance to it.
To put it in the language more familiar with the ancient Church, the Christian has Christ as King and Lord, and that means an implicit, and even explicit, rejection of the lordship of Caesar. Jesus Christ is Lord, not Caesar. We do not offer a pinch of incense and invoke the name(s) of false gods, even if we are threatened with the sword.
And speaking of the sword, the State has a valid license to use the sword, as St. Paul says in Romans 13. And Christians, therefore, have a vocation as citizens/subjects to live peaceably with others, to pay our taxes, and to do what is honorable in the sight of all (see Romans 12 and 13 together).
Seeing, then, also that the State has no right to tell us what to do; neither does the Church bear a right to command the State. That is the doctrine of the two kingdoms in action. Though Bonhoeffer expresses the difficulty of maintaining this matter in his essay "The Church and the Jewish Question", in which he emphasizes the importance of the two kingdoms theology but also seeks to address how and where the Church must act and speak up. The context of the essay was the fact that the Nazis and their allies in the churches sought to deny Jewish Christians their rightful place in the Church as baptized members of the Household of God. This was simply unconscionable, unacceptable, and presented sufficient reason for the Church to act. Not because this was simply one step too far by the Nazis as though everything else they did wasn't sufficiently evil (it certainly was) but rather the Church's hand was now being forced to move, and if necessary, to wrest the wheel away lest the Church herself be crushed.
"The church must repudiate such an encroachment by the state authorities, precisely because it knows better about the state and the limits of its actions. A state that threatens the proclamation of the Christian message negates itself. There are thus three possibilities for action that the church can take vis-à-vis the state: first (as we have said), questioning the state as to the legitimate state character of its actions, that is, making the state responsible for what it does. Second is service to the victims of the state’s actions. The church has an unconditional obligation toward the victims of any societal order, even if they do not belong to the Christian community. “Let us work for the good of all” [Gal. 6:10]. These are both ways in which the church, in its freedom, conducts itself in the interest of a free state. In times when the laws are changing, the church may under no circumstances neglect either of these duties. The third possibility is not just to bind up the wounds of the victims beneath the wheel but to seize the wheel itself. Such an action would be direct political action on the part of the church. This is only possible and called for if the church sees the state to be failing in its function of creating law and order, that is, if the church perceives that the state, without any scruples, has created either too much or too little law and order. It must see in either eventuality a threat to the existence of the state and thus to its own existence as well. There would be too little if any one group of citizens is deprived of its rights. There would be too much in the case of an attack, coming from the state, on the nature of the church and its proclamation, such as the obligatory exclusion of baptized Jews from our Christian congregations or a ban on missions to the Jews. In such a case, the church would find itself in statu confessionis, and the state would find itself in the act of self-negation." - Dietrich Bonhoeffer, The Church and the Jewish Question, The Bonhoeffer Reader, Kindle Edition
And so I argue thus: It is of great importance that the State neither take up the occupation of the Church; nor the Church the occupation of the State. Nevertheless, the Church must at all times be herself, and to be herself free. And thus the Church must not, and cannot, align herself nor conjoin herself with political power, such as by becoming the instrument of a political party or particular faction of political interest. In doing so she denies herself, she denies her mission, her proclamation, and her Lord. And to be free she must retain her total allegiance to Jesus Christ and at all times proclaim the word of God, and never at any time become servile to the whims of any political power; for neither prince, nor potentate,, nor parliament, nor party are lord over her; for she knows one Lord and one God; and she is of but one Faith, having one baptism, and is one Body.
And the problem with overt nationalism is quite simply thus: It is to sell the Church over to the princely powers of the world and to deny her divine purpose and mission as the Bride and Body of the Bridegroom. We can be good citizens and subjects, and we ought to be, for the good of our neighbors. But we are not beholden to the worship of idols, to confess the lordship of any Caesar, or to turn ourselves away from the Gospel of Jesus Christ and preach another gospel, for that would be anathema and a denial of Jesus Christ Living and Incarnate.
-CryptoLutheran