The Church exists in the World. But how should it relate to the World?
In some times and places, the Church has seemed indistinguishable from the World. The Church may believe it is influencing the World, but often it is only being corrupted and/or used by it.
During other eras, the Church has ignored the World, turning its back on its needs. The Church has become an insulated, isolated otherworldly club - and the World is left to fend for itself.
In II Corinthians 6, Paul advocates for a definite separation between the Church and the World. In vv. 14-18, he stresses that there is no commonality, no union, between the things of Christ and the things of demons. It is a helpful reminder that we are called to stand apart from the world, that there should be a distinction between Christians and non-Christians - not that we are better, but that we are striving to follow God in every way.
Yet the call to be separate does not involve a wholesale leaving and rejection of participation in the World. Paul also wrote I Corinthians 5:9 where he says, "I have written you in my letter not to associate with sexually immoral people, not at all meaning the people of this world who are immoral, or the greedy or swindlers, or idolaters. In that case you would have to leave this world." This is not Paul's command or even his suggestion in II Corinthians 6 but a reminder that the Church needs to guard its ways very carefully when it comes to interacting with the World. Too much is at stake!