Good quote about christians in the world from The So-called Letter to Diognetus on CCEL.org
To put it simply: What the soul is in the body, that Christians are in the world. The soul is dispersed through all the members of the body, and Christians are scattered through all the cities of the world. The soul dwells in the body, but does not belong to the body, and Christians dwell in the world, but do not belong to the world. The soul, which is invisible, is kept under guard in the visible body; in the same way, Christians are recognised when they are in the world, but their religion remains unseen. The flesh hates the soul and treats it as an enemy, even though it has suffered no wrong, because it is prevented from enjoying its pleasures; so too the world hates Christians, even though it suffers no wrong at their hands, because they range themselves against its pleasures. The soul loves the flesh that hates it, and its members; in the same way, Christians love those who hate them. The soul is shut up in the body, and yet itself holds the body together; while Christians are restrained in the world as in a prison, and yet themselves hold the world together. The soul, which is immortal, is housed in a mortal dwelling; while Christians are settled among corruptible things, to wait for the incorruptibility that will be theirs in heaven. The soul, when faring badly as to food and drink, grows better; so too Christians, when punished, day by day increase more and more. It is to no less a post than this that God has ordered them, and they must not try to evade it.