And this is God’s plan: Both Gentiles and Jews who believe the Good News share equally in the riches inherited by God’s children. Both are part of the same body, and both enjoy the promise of blessings because they belong to Christ Jesus
Ephesians 3:6-7

And there you have it: God’s plan.

• Make every effort to keep yourselves united in the Spirit, binding yourselves together with peace
  Ephesians 4:3

Paul is instructing us to work to keep unity, not create it. Unity among Believers was established by God.

• For there is one body and one Spirit, just as you have been called to one glorious hope for the future.
  Ephesians 4:4

So why isn’t there unity among the body of Christ.

We must endeavor to keep this unity – we do not create it. God never commands us to create unity among believers. He has created it by His Spirit; our duty is to recognize it and keep it.

Spurgeon suggests the idea of spiritual corporations causes division. (I hope you are sitting down.) And Frederick Bruce had some thoughts, too.

• “It is not a desirable thing that all Churches should melt into one another and become one; for the complete fusion of all Churches into one ecclesiastical corporation would inevitably produce another form of Popery, since history teaches us that large ecclesiastical bodies grow more or less corrupt as a matter of course. Huge spiritual corporations are, as a whole, the strongholds of tyranny and the refuges of abuse; and it is only a matter of time when they shall break to pieces.” – Charles Spurgeon

• “For the church fellowship in which the Gentile and Jewish believers were united was no mere enrollment on a register of membership; it involved their union with Christ by faith and therefore their union with each other as fellow-members of his body.” – Frederick Bruce

• “We want unity in the truth of God through the Spirit of God. This let us seek after; let us live near to Christ, for this is the best way of promoting unity. Divisions in Churches never begin with those full of love to the Savior.” – Charles Spurgeon

Unity of the Church isn’t tied to membership of a church, devotion to denomination decrees and rituals, but to the seeking of Christ, and the living near to Christ. When all Believers do this, the unity that Christ created is easy to keep.

Seek to live the way I’m supposed to live.