The oneness of the Body of Christ is the oneness of the Spirit (Eph. 4:3) and the oneness of the divine constitution by the Divine Trinity (John 17:21-23). This is the constitutional aspect of the unique oneness for the church ground. This divine oneness separates all the believers in Christ from the unbelievers and unites all the believers together to be the one Body of Christ.
There is also the practical aspect of the oneness of the church for the church ground. All the believers in Christ are the components of the Body of Christ. Practically, they are scattered in many cities on this earth. Spontaneously, they are separated into many units in each city, respectively, according to their dwelling. According to the New Testament pattern set up and ordained by God and according to the principle of the New Testament revelation concerning God’s economy of the church, in each city in which the believers dwell, it is not allowed to have more than one unit as a local expression of the unique Body of Christ, nor is it permitted to have a local church in a certain city without the proper fellowship in the Body of Christ with the other local churches. All these limitations of the church ground in its oneness are the safeguard in the church life to avoid any kind of division in the Body of Christ.
The local ground of the church is basically the unique oneness of the Body of Christ practiced in the local churches. Both the universal Body of Christ and the local churches are uniquely one. There is one unique Body of Christ in the whole universe, and there is one unique local church in each locality, respectively. This unique oneness is the basic element of the church life. Since the oneness of the Body of Christ is the oneness of the Spirit (Eph. 4:3), the oneness practiced in a local church must be in the move of the Spirit and under the government of the Spirit. Hence, this Spirit is also a basic element of the church ground. In addition to this, since a local church is very much involved with its locality, the locality of the local church is also a crucial element of the church ground. So, the church ground on which a local church is built must be constituted with and prevail in the oneness executed by the Spirit and the oneness safeguarded by the locality.
In addition to the oneness of the Spirit and the oneness practiced in the locality of a local church as mentioned above, there is another aspect of the oneness, which is "the oneness of the faith and of the full knowledge of the Son of God," as revealed in Ephesians 4:13. Concerning this, note 132 in Ephesians 4, Recovery Version, says, "In v. 3 the oneness of the Spirit is the oneness of the divine life in reality; in this verse the oneness is the oneness of our living in practicality. We already have the oneness of the divine life in reality. We need only to keep it. But we need to go on until we arrive at the oneness of our living in practicality. This aspect of oneness is of two things: the faith and the full knowledge of the Son of God. As revealed in Jude 3, 2 Tim. 4:7, and 1 Tim. 6:21, the faith does not refer to the act of our believing but to the things in which we believe, such as the divine person of Christ and His redemptive work accomplished for our salvation. The full knowledge of the Son of God is the apprehension of the revelation concerning the Son of God for our experience. The more we grow in life, the more we will cleave to the faith and to the apprehension of Christ, and the more we will drop all the minor and meaner doctrinal concepts that cause divisions. Then we will arrive at, or attain to, the practical oneness; that is, we will arrive at a full-grown man, at the measure of the stature of the fullness of Christ." To keep this aspect of the oneness we need to endeavor to "drop all the minor and meaner doctrinal concepts [the winds of teaching] that cause divisions. Then we will arrive at, or attain to, the practical oneness; that is, we will arrive at a full-grown man, at the measure of the stature of the fullness of Christ."
(The Crucial Points of the Major Items of the Lord's Recovery, Chapter 1, by Witness Lee)