That they may all be one; even as thou, Father, art in me, and I in thee, that they also may be in us. John 17.21.
Quite simply, oneness is God himself. When all of us set aside the things outside of God and begin to live in Him, then God who is in us becomes the oneness. Oneness is when God has His absolute place in us. Oneness is when He alone is in all, when He fills all. When the children of God are filled with God, they harmonize with one another. As a matter of fact, Satan, in his attempt to effect the disintegration of us as a body, does not need to stir up opinions and strife among us so long as he is able to plant some impurity in us or something else which takes the place of God. As an illustration of this, have you ever noticed how people mix concrete? If there is some clay blended with the sand, the cement will not firmly congeal. Now for Satan to destroy our oneness in the body, he needs to do nothing but spread a little mud—that which is incompatible with the life of God in us—and we as a body shall disintegrate. We have but one need, which is, to turn inwardly to God and let Him cleanse us and purify us with the cross and the Holy Spirit.