The City of God

I just listened to an excellent edition of The White Horse Inn, that were it in my power to do so, I would want to sit you all down and make you listen to it. (You can find it at The White Horse Inn on One Place.)
I would also beg you to read an article by Mike Horton entitled, "Augustine & Jerome."
On the program, in talking about Augustine's The City of God, the guys say:

Augustine didn't necessarily see the barbarians who attacked Rome as enemies of The City of God. He didn't se them as Jerome did: pagan enemies of God. He said, we don't know who the elect are and so maybe they are "citizens to be" of the Kingdom of God. Instead, let's reach out to them as missionaries would and as friends. "After all, man's true end," Augustine argued, "lies beyond this life. The Churcn exists for the Kingdom of Heaven and God knows the elect, so the meaning of history lies, not in the flux of outward events, but in the hidden drama of sin and the redemption."
In church history, they said, often its when the church just does what it is supposed to do that it inadvertently ends up transforming the culture as a side affect.

In the late 1st and early 2nd centuries, Justin Martyr entered into an apologetic defense of the faith with a Jew named Trypho. (See this dialogue at St. Justin Martyr Dialogue with Trypho.)
In part, we read

"This is what we are amazed at [about you Christians]," said Trypho. "... I am aware that your precepts in the so-called Gospel are so wonderful and so great, that I suspect no one can keep them; for I have carefully read them. But this is what we are most at a loss about: that you, professing to be pious, and supposing yourselves better than others, are not in any particular separated from them, and do not alter your mode of living from the nations, in that you observe no [pagan] festivals or [Jewish] sabbaths, and do not have the rite of circumcision; and further, resting your hopes on a man that was crucified, you yet expect to obtain some good thing from God, while [in spite of the fact that] you do not obey His commandments."

I hope you take time to listen to this particular program and, perhaps, wrestle with it. We are the Church. We are citizens of Heaven and, at the same time, of the city in which we live. We are called to live among those outside the church and be good neighbors.

Later in the program, they point out Jeremiah 29:3-7, which says, "The letter was sent by the hand of Elasah the son of Shaphan and Gemariah the son of Hilkiah, whom Zedekiah king of Judah sent to Babylon to Nebuchadnezzar king of Babylon. It said: "Thus says the LORD of hosts, the God of Israel, to all the exiles whom I have sent into exile from Jerusalem to Babylon: Build houses and live in them; plant gardens and eat their produce. Take wives and have sons and daughters; take wives for your sons, and give your daughters in marriage, that they may bear sons and daughters; multiply there, and do not decrease. But seek the welfare of the city where I have sent you into exile, and pray to the LORD on its behalf, for in its welfare you will find your welfare."