Psalm 47:7-8 New International Version
7 For God is the King of all the earth;
sing to him a psalm of praise.
8 God reigns over the nations;
God is seated on his holy throne.
God as "King of all the earth" is a foundational aspect of the Judeo-Christian worldview. God has no rivals and certainly no equals. The position and dominion of God is secure, always has been, always will be. And yet, 'Christian' Nationalism brings the Kingdom of God down into the trenches, pitting nation against nation, viewing the rise of America (or England, the Holy Roman Empire, Rome, etc. before us) as the flowering of God's purposes, and the potential fall of the favored nation as the withering of God's will. How can this be? Does God not rule the whole earth? Is his purpose not accomplished in every nation?
One of the reasons why 'Christian' Nationalism fails to see the big picture is egocentrism, the belief that God's will must revolve around us and our nation because we're just that important (as compared to other peoples and nations of the world, whom God evidently cares much less about). This was, at times, a flaw of ancient Israel as well, one that Holy Scripture punctures in the story of the prophet Jonah. The entire book of Jonah is a withering rebuke of Israelite nationalists whose concern about humanity stopped at the boundary of their own ethnicity. God's concern did not end there, however. As the prophet Jonah throws a tantrum while watching God's mercy against Nineveh unfold, the book ends with this line:
Jonah 4:11 New International Version
And should I not have concern for the great city of Nineveh, in which there are more than a hundred and twenty thousand people who cannot tell their right hand from their left—and also many animals?”
The view of Jonah was far too narrow for God, spending three nights in the belly of the great beast of the sea didn't cure him of his folly, so the book ends with us wondering if he hardened his heart following God's rebuke, or repented.
Likewise, the Gospels contain numerous examples in the actions and words of Jesus that refute the notion that God's focus on one nation means that others fall through the cracks. Two prime examples being Jesus' discussion with the Woman at the Well and the Parable of the Good Samaritan.
Lastly, there is one caveat to this discussion, one that will be dealt with numerous times in the texts yet to come in this series: Israel had a covenant with God. Abraham's descendants did have a relationship with God that held promises of great favor AND a stricter level of scrutiny regarding their actions. Israel is the exception to the rule, but as Jonah learned the hard way, the special status of Israel did not remove the other nations of the world from God's concern.
In the end, the Church is NOT Israel, and America is NOT Israel. Whatever promises God made to Israel remain with them, they are not transferable. 'Christian' Nationalism fails, in part, because it seeks to focus God's concern on one nation (the one we happen to live in, imagine that), viewing the world in terms of power politics and national rivalries as they impact that one nation. God's concern is higher, deeper, and much broader.