Flags in Church

I have always been part of a church, at least in my adult life, that has displayed the American flag, the state flag, and a Christian flag, at the very least, in the church. On another platform discussion, someone called displaying the American flag in church as idolatry. Their argument was that our ultimate allegiance is the Kingdom of God and not a country. I agree that America is not where our ultimate citizenship is, but does displaying the flag really equate to idolatry? Thoughts? What do y'all do at your churches?