Watchman Nee saw this truth about one church in every city and wrote about it extensively.
"The city is the smallest geographical unit upon which a local church can be formed; anything smaller is not scriptural. Watchman Nee uses the apostle Pauls rebuke of the believers in Corinth in 1 Corinthians chapter 1 to show that the biblical standard does not permit believers to divide themselves into smaller groups. Such groups are not real local churches, but sects:
We read in 1 Corinthians 1:2 of the church of God which is in Corinth. Corinth was a unit-locality, and the church in Corinth, a unit-church. When discord crept in and its members were on the point of splitting the church into four different factions, Paul wrote, rebuking them: Each of you says, I am of Paul, and I of Apollos, and I of Cephas, and I of Christ
Are you not men of flesh? (1 Cor. 1:12; 3:4). Had these people formed four different groups, they would have been sects, not churches, for Corinth was a city, and that is the smallest unit which warrants the forming of a church. The church of God in Corinth could not cover a lesser area than the whole city, nor could it comprise a lesser number of Christians than all the Christians who lived there. This is Pauls definition of the church in Corinthto those who have been sanctified in Christ Jesus, the called saints (1:2). To form a church in an area smaller than a unit-locality is to form it on a smaller basis than a scriptural unit, and it follows that it cannot be a scriptural church. Any group of believers less than all the believers in a place is not qualified to be a separate church. The unit of the church must correspond with the unit of the locality. A church must cover the same area as the locality in which it is found. If a church is smaller than a locality, then it is not a scriptural church; it is a sect.
To take any name other than Christs and use that designation as a basis for division among the believers is condemned by the apostle Paul as carnal, fleshly, and sectarian. However, even to confess, I am of Christ, if done with a divisive attitude, is equally condemned. This is because, as Watchman Nee reminds us, the only scriptural ground for a local church is locality, not sectarianism or even non-sectarianism:
To say, I am of Paul, or I am of Cephas, is obviously sectarian; but to say, I am of Christ, is sectarian too, though less obviously so. The confession, I am of Christ, is good as a confession, but it is not an adequate basis for forming a separate church, since it excludes some of the children of God in a given locality by including only a certain section who say, I am of Christ. That every believer belongs to Christ is a fact, whether that fact be declared or not; and to differentiate between those who proclaim it and those who do not, is condemned by God as carnal. It is the fact that matters, not the declaration of it. The sphere of a church in any place does not merely include those in that place who say, I am of Christ, but all in that place who are of Christ. It extends over the entire area of the locality, and includes the entire number of the Christians in the locality.
To take ones stand as belonging to Christ alone is perfectly right, but to divide between Christians who take that stand and Christians who do not, is altogether wrong. To brand as sectarian those who say, I am of Paul, or I am of Cephas, and feel spiritually superior as we separate ourselves from them and have fellowship only with those who say, I am of Christ, makes us guilty of the very sin we condemn in others. If we make non-sectarianism the basis of our fellowship, then we are dividing the church on a ground other than the one ordained of God, and thereby we form another sect. The scriptural ground for a church is a locality and not non-sectarianism. Any fellowship that is not as wide as the locality is sectarian. All Christians who live in the same place as I do, are in the same church as I am, and I dare exclude none. I acknowledge as my brother, and as a fellow member of my church, every child of God who lives in my locality.
Emphatically reiterating that there can be only one local church in one city, Watchman Nee demonstrates from Scripture that regardless of the size of the city or the number of believers in that city, if the locality is indivisible, so the local church in that locality is indivisible:
There were a great number of believers in Jerusalem. We read of a multitude who turned to the Lord; yet they are all referred to as the church in Jerusalem, not the churches in Jerusalem. Jerusalem was a single place; therefore, it could only be counted as a single unit for the founding of a single church. You cannot divide the church unless you can divide the place. If there is only one locality, there can only be one church. In Corinth there was only the church in Corinth; in Hankow there is only the church in Hankow. We do not read of the churches in Jerusalem, or the churches in Ephesus, or the churches in Corinth. Each of these was counted as only one place; therefore, it was permissible to have only one church in each. As long as Jerusalem, Ephesus, and Corinth remain unit-localities, just so long do they remain unit-churches. If a locality is indivisible, then the church formed in that locality is indivisible."