As the body is one, and hath many members, ... so also is Christ.
He is the head of the body, the church. — The head over things to the church, which is his body, the fulness of him that filleth all in all. — We are members of his body, of his flesh, and of his bones.
A body hast thou prepared me. — Thine eyes did see my substance, yet being unperfect; and in thy book all my members were written, which in contin-uance were fashioned, when as yet there was none of them.
Thine they were, and thou gavest them me. — He hath chosen us in him before the foundation of the world. — Whom he did foreknow, he also did predestinate to be conformed to the image of his Son.
Grow up into him in all things, which is the head, even Christ: from whom the whole body fitly joined together, and compacted by that which every joint supplieth ... maketh increase of the body unto the edifying of itself in love.
We have been ordained to be one in HIm. that's why there is always a common good and a common goal in the church, and why we always feel a kinship with other true Christians we meet. We belong together to the Lord Jesus, together, united both with Him and with other believers. That is the doctrinal part. Practicing the doctrine is a bit different, as reality often tells us; while we are all "one," still we often treat other believers as worthless, just because they are of a different "tribe" than we are (Baptists vs. Pentecostals vs. Catholics vs...); individually, we allow ourselves to bring ordinary human society into the oneness we have in Christ, so that we criticize, hate, reject, cheat, become arrogant, all against those whom Jesus said, "Love one another." The final set of verses above shows a different way--"grow up into Him in all things"--in other words, strive to become like our Lord Jesus in all ways. This actually promotes the unity and closeness of the entire church, and fills it with love.
I COR. 12:12. Col. 1:18. -Eph. 1:22,23. ‑Eph. 5:30. Heb. 10:5. ‑Psa. 139:16. John 17:6. -Eph. 1:4. -Rom. 8:29. Eph. 4:15,16.