Doug Wilson is again helpful on this subject (here’s my part I linking to one of his posts from April):
When we consider the temptations of nationalism, we have to think through how conservative Christians might be drawn into that error — and it is an error.
We are first presented with a false alternative. On the one hand, we see those who despise America and its symbols. Think flag-burning, those who defend flag-burning, and all that. And then on the other hand, think of those believers who react against this and are lured into a syncretistic civil religion, in which a red, white, and blue cloth takes it’s place as a third sacrament. This is, speaking theologically for a moment, demented. And yet it is “justified” because flag burning is not honoring the emperor, like the Bible says to do, and this syncretism is not flag burning. Ta da!
But there is another alternative. It is possible to render subordinated honor that is not idolatrous. Not only is it possible to do, it is necessary. But in order for this honor to be a biblical civil honoring, words like sacred, hallowed, and religion must be kept entirely and completely away from it. There must be a sharp line of demarcation that separates the kind of honor that is due to a nation and its magistrates and the kind of honor that belongs to God alone. That line of demarcation must be maintained by the kinds of words we use and will not use.
I should also note that Wilson considers the temptation that conservative Christians have towards nationalist idolatry to be less dangerous than support for government activity that he believes to be far in excess of government’s biblical and constitutional authority. Statist idolatry, according to Wilson, is far more dangerous and far less noticed than syncretic God-and-country nationalism.