Fundamentally, the church, as the unique Body of Christ, is one universally and includes every believer irrespective of time and space. Practically, this one universal Body of Christ is manifested in time and space in various cities as local churches. These local churches include every believer in that city at that time and also maintain a fellowship with all the other local churches, thus practically expressing the one Body of Christ.
To maintain this oneness, two crucial matters are needed. On the one hand, a local church must receive all those whom Christ has received (Rom. 15:7). On the other hand, a local church should not receive any teaching other than the apostles’ teaching in the New Testament (1 Tim. 1:3-4). Practically that means not receiving a ministry that teaches things that are different from the apostles’ teaching, including teaching the New Testament revelation selectively. This article on Shepherding Words shows that receiving different teachings inevitably damages the oneness of the Body of Christ and leads to division. These two principles—receiving all believers and refusing different teachings—must both be upheld to preserve the local churches in the proper oneness.