It is such a common phrase, and so widely accepted in American Christianity, that I long believed it and even said it.
But I was wrong.
I noticed one day that I never actually see that called out anywhere in Scripture. I never see a Scripture quoted when someone says that.
Instead, the Scripture actually says the exact opposite.
"[People of this world's] destiny is destruction, their god is in their stomach, and their glory is in their shame. But our citizenship is in heaven. And we eagerly await a Savior from there, the Lord Jesus Christ, who by the power that enables Him to bring everything under His control, will transform our lowly bodies so that they will be like His glorious body." (Phil 3:19-21)
Paul says here that our citizenship is in heaven as opposed to those around us, and we eagerly await the One who will come and set things right. The picture is of us as foreigners in an evil land.
"Dear friends, I urge you, as foreigners and exiles...live such godly lives among the pagans that, though they may accuse you of doing wrong, they may see your good deeds and glorify God on the day He visits us. Submit yourself for the Lord's sake to every authority...live as God's slaves. Show proper respect to everyone, love the family of believers, fear God, and honor the emperor." (1 Pet 2:11-17)
Peter here says that we are not citizens here, but are exiles and foreigners. Despite this we should follow the laws of the land we are visiting and show proper respect to everyone, while still living as God's slaves and not as the world around us. Romans 13 and Titus 3 both argue the same: that we should submit to the leaders of our kingdom. Why do they need to say that? Because early Jews and Christians who were serious about their faith universally rejected the evil rule of the idolatrous Roman Empire, and never would have seen themselves as "co-citizens." Instead, they had to be reminded not to start riots and live in peace with their evildoing neighbors.
"Then give back to Caesar what is Caesar's and give to God what is God's." (Luke 20:25)
When the Pharisees tried to trap Jesus into either denouncing Caesar or admitting to following Caesar, He did neither. He refused to claim citizenship with Caesar, but also refused to denounce Caesar's right to rule. Instead He said that we should pay Caesar the taxes we owe him, but give to God the allegiance which we owe Him.
Implied within this statement, I believe, is the clear message that the only reason they were under Caesar's rule to start with was their unwillingness to follow God and His Kingdom.
"We must obey God rather than men." (Acts 5:29)
Not much commentary to add on that one.
The point is...we are NOT citizens of America and citizens of heaven. A man cannot serve two masters--if he tries to serve one he will end up hating the other, and vice versa. (Someone wise once said that...)
In the same way, we must follow what the Bible teaches. We are citizens of heaven, foreigners here on earth for a while. Yes we should follow the laws insofar as we are able, and yes we should seek peace. But that in no way means that you are a citizen of this earth.
I've written about the politicizing of Christianity by American politics before, and I probably will again. We American Christians tend to be Americans first, and Christians when convenient. Need proof? Think of how many white American Christians are FURIOUS about the portrayal of the police in Ferguson, compared to how many simply ignored the beheadings going on in Iraq. Why? Because the American police and soldiers are the martyrs and saints of Americanism religion, and if you get more upset about their mistreatment than the mistreatment of poor Middle Eastern Christians, then guess what? You aren't serving the right Master.
We are foreigners, not dual citizens. Never forget that. Do what the Bible tells us to do--honor your leaders and pray for them. Follow their laws as long as you are able. But don't ever start to think that their kingdom somehow is "co" God's kingdom when it comes to your citizenship.