This is clear from the name by which God's people are called in the New Testament. We are the church: ekklesia - the called out ones. We are called out of the world and added to the church. Summoned by sovereign decree to leave one reality and recreated by grace to be part of the alternative reality.
It is equally clear the other way round. In those rare sad instances when persistence in known sin makes a person incompatible with continuing in the life of the church, the only biblical option is excommunication. This means that people who were once part of the church leave that reality and thereby find themselves in the only other reality: back in the world.
Either way, it is in or out. There is no neutral gray area between the two realities, and no way to straddle the fence.
We are not talking about where we live. Jesus was clear that the church was to continue to be "in the world". The dividing line between the church and the world is not a geographical boundary as if we could cross back and forth over the border between two nations, between two sovereign states. In the church on Sunday and in the world on Monday. If that were the case, perhaps dual citizenship would be possible. It's about identity rather than geography. In the world (in terms of geography), but not of the world (in terms of identity). Having been called out from the world, "church" is now who we are. Being born again is becoming an entirely new creation; it is being given a new identity with no turning back as the old things have passed away and everything has become new. That identity is in Christ; but you cannot be in Christ without being in his body, for he will never be beheaded.
We still think of church as where we go (a building) or what we do (activities and services). But church is who we are. If we really understood who we are, we would be a lot clearer about what we are not. We are not the world. Knowing who we are is the root out of which where we go and what we do will grow. We are known by our fruit. It is clearly visible in where we go and what we do. And distinguishing church people and world people should be as simple as comparing apples and oranges. There are no appanges.