Just Don't do it
Douglas Wilson is pastor of Christ Church in Moscow, Idaho, and author of many books, including Letter from a Christian Citizen (American Vision, 2007).
A Christian church has absolutely no business displaying a national flag in the sanctuary, at least not as it is commonly done. The church born at Pentecost was a reversal of Babel, not a doubling down on the fragmentation of Babel.
Our churches should not place any unnecessary barriers to the worship of visiting Koreans, Russians, or Portuguese. We already must deal with natural and providential barriers, such as differences in language. In Christ there is neither Jew nor Gentile (Gal. 3:28). I wouldn't want to worship in a sanctuary with a Scythian flag up front (Col. 3:11), and the Golden Rule requires that we not do to the visiting Scythians what we didn't very much like when they did it to us.
The New Testament is all about this principle. Where customs interfere with transnational fellowship, those customs must give way (Acts 15:29).
Of course, I do want to note that this should be taken as a general principled stand, which is not the same as a perfectionistic one. I am not interested in applying any strictures to churches that are renting space in American Legion halls or in high school gyms, where a flag would already be displayed in the ordinary course of things. I am simply arguing that the flag up front should not be part of how the church as the church arranges things. When we in the church have a decision to make, we shouldn't decide in favor of displaying national flags.
If the church places an American flag in the front of the sanctuary, this becomes part of our sacred architecture, and therefore says something. It becomes a shaping influence. ...