We were born into the oneness in reality. Then we need to arrive at the oneness in practicality. There is a process from the oneness in reality to arrive at the oneness in practicality (Eph. 4:12-16). To fully arrive at the oneness in practicality, we need to be perfected by the gifts unto the work of the New Testament ministry for the building up of the Body of Christ (v. 12). Then we will grow from babyhood to a full-grown man, no longer being tossed by the waves, the turmoils, and no longer being carried about by every wind of teaching, doctrine (v. 14). To arrive at the oneness in practicality, we need to hold to truth in love to grow up in all things into the Head, Christ (v. 15). Eventually, we will be a full-grown man with the measure of the stature of the fullness of Christ, the Body of Christ (v. 13). We will be built in the Body of Christ by being joined together through every joint of the rich supply of Christ and by being knit together through the operation in the measure of each one part of the Body (v. 16). This is the best and most effective way to keep the oneness of the Body (vv. 2-3).
The apostle Paul spoke of the Body of Christ and warned the believers about division damaging the oneness of the Body. In Romans, Paul mentioned the Body in chapter twelve, saying that "we who are many are one body in Christ" (v. 5). The following book is 1 Corinthians. First Corinthians is a rich book on the enjoyment of Christ. In 1 Corinthians Christ is revealed as twenty items for our enjoyment (see note 92 in 1 Corinthians 1) Eventually, 1 Corinthians reveals that such a One became the life-giving Spirit. This is the twentieth item. All the foregoing nineteen items are included in this last item. If He were not the life-giving Spirit, He could never be our wisdom: our righteousness, our sanctification, and our redemption (v. 30). If He were not the life-giving Spirit, He could not have come into us, and He would have nothing to do with us in our experience. But thank Him that He is the life-giving Spirit. Furthermore, 1 Corinthians 6:17 says, "He who is joined to the Lord is one spirit." We are one spirit with Him because He is the Spirit mingled with our spirit. We have an organic union with Him in our spirit.
First Corinthians is such a marvelous book on the enjoyment of Christ, but this book also covers the negative side. It teaches us how to face division, how to deal with division, and how to avoid division. Ephesians teaches us mainly about the oneness on the positive side, whereas 1 Corinthians teaches us mainly about the oneness on the negative side.