As we read the beginning of Paul's letters, we clearly see that he writes to churches (unless he is writing to close associates such as Timothy).
Paul usually writes something like, "To the church in..." or "To the saints in..." or some variation of this.
I have recently realized that what I have pictured in my mind and what Paul meant were probably two very different things.
As I have read Paul's letters over the years, I have pictured him writing to a local body. In particular, I have pictured those who are in the church (of some city) to the exclusion of those who aren't in the church. I suppose I was inadvertently taking the modern view of church membership and placing it upon those churches. For example, I was picturing Paul writing to the members of the church in Rome, while not writing to those who were not members.
As I read Paul's letters today, I think the apostle was emphasizing something else when he wrote, "To the church in..." Paul was making it clear that he was writing to ALL the saved people/followers of Jesus/Christians who happened to reside in a particular city. Paul had no aim of excluding any Christian from hearing the letter, but was simply addressing it to those who were in the church body in a particular geographic region.
Common sense tells us this must have been the case. Not everyone in these cities would have met in the same house. Therefore, the letters must have been circulated around the city. In fact, all the followers of Jesus in a particular city seem to have been considered part of the church in that city. Additionally, some of these churches were located relatively far apart, such as those who received the letter to the Galatians. It also appears that some of Paul's letters were circulated among churches who were not even named in the greeting, such as the church in Laodicea receiving the letter to the Colossians. Colossians 4:16 tells us, "Now when this epistle is read among you, see that it is read also in the church of the Laodiceans, and that you likewise read the epistle from Laodicea."
I write all this to make the point that Paul did not seem particularly concerned about boundaries for churches. He wrote to followers of Christ who lived in certain areas, but his focus hardly seemed to be aimed at writing to some to the exclusion of others.
This ought to make us think about how we view the church. If Paul wrote to all the Christians in an area, it is clear that he considered them ALL to be part of the church. Paul did not make the universal/local distinction that we tend to make in the modern church. If Paul saw, for example, all the Christians in a city as part of the church of that city, that should inform us as we think about issues of unity and membership.
"To the church in..." suggests unity as opposed to exclusion.