"There is one body," not a philosophical or natural entity but a mystical one-- the church.
The cords of love are not woven in nature's loom; they are textured only by divine revelation. Thus Paul confidently exhorts Christians "to keep the unity of the Spirit in the bond of peace" Ephesians 4:3.
The apostle then reminds God's people that "there is one body," not a philosophical or natural entity but a mystical one-- the church-- which consists of many saints (V. 4). If it is not normal for one member of a man's body to battle another, when they are all preserved in life by their union together, so much less in the mystical body.
Again, there is "one Spirit" who quickens all true saints, and He is to the whole body of Christians as the soul is to the whole man (v. 4). Now it is a strange violence against nature for members of a man's body to war against one another and drive out the soul, which gives life to them in their oneness. Surely it would be even more perverse for Christians to force the Holy Spirit to leave because of their contention and strife. A wider door cannot easily be opened for Him to go.
Further, the apostle persuades Christians to preserve unity because of the "one hope of your calling"-- the bliss which we all hope for in heaven (v. 4). There is a day coming-- and it cannot be far away-- when we shall lovingly meet in heaven and sit at the same feast together, with no envy at what is on our neighbor's plate. Full fruition of God will be the feast, and peace and love the sweet music that accompanies it. How senseless it is, then, for us to fight on earth when we shall feast together in heaven. Now it is the Gospel, which invites us to this feast and calls us to this unity. Other truths are engraved on the same holy invitation and command-- "one Lord, one faith, one baptism" (v. 5)-- but I shall leave these for your own study.”Quoted material from, ”The Christian in Complete Armour Daily Readings in Spiritual Warfare” by Gurnall and James S Bell. http://www.moodypublishers.com/pub_productDetail.aspx?id=41823&pid=53617