Nalanda temple, Bihar, India. This ancient seat of learning
flourished between the 5th and 12th centuries CE and is described
as "one of the first great universities in recorded history."
Universities are, perhaps, one of the oldest examples of this phenomenon: pack a lot of capable people together and they come up with so much more than any of them would achieve on their own.
The arts, culture, commerce, the intellect, engineering, cooking and, inevitably, crime all flourish in cities in ways that would never be achieved in an agrarian society.
The church has been part of and has benefited from this flourishing. As beneficiary it should thank God. It is enjoined to pray for the peace of Jerusalem (Ps 122:6) and those exiled from Jerusalem to Babylon were told to seek its peace and prosperity. (Jer 29:7). More to the point, the church has no choice. Almost everywhere is city or has been landscaped by the demands of the city (for food, water, power, labour, communications). It is inescapable.
The art or goal, it seems to me, is for the church collectively and faithful Christians individually to be in-and-apart-from the city. For most faithful people, I suggest, holy living is not a search for a world-denying asceticism but is a struggle to be both Christ-centred and engaged in the quotidian business of making a living, sharing family life, growing up and growing old. Christians are normal, but they choose in some way to set themselves apart.
I don't think there are any easy answers to the immediate questions: what might a proper 'apart' be - and what would be the criteria by which to judge - and how can the separateness be a positive element in both personal growth and community engagement? These are questions of identity, to be lived out in practice in changing circumstances, characterised by judgements of value rather than doctrine, and all but impossible to determine in the abstract.
To stand apart can (should) give a critical distance by which Christians may both critique the secular world and contribute constructively to it; to do so while knowing that we are ineluctably part of the secular world should also place a critical brake on the tendency to self-righteousness and complacency.
I guess the only way is to keep practising.