In the early church, Christians never had any doubt that they must be different from the world; they, in fact, knew that they must be so different that the probability was that the world would kill them and certainly was that the world would hate them. But the tendency in the modern Church has been to play down the difference between the Church and the world. We have, in effect, often said to people: ‘As long as you live a decent, respectable life, it is quite all right to become a church member and to call yourself a Christian. You don’t need to be so very different from other people.’ [When] in fact, Christians should be easily identifiable in the world. Christ does not take us out of the world; He makes us different within the world.
– William Barclay –
from his commentary on Ephesians 1.3-4