He is the image of the invisible God, the firstborn of all creation.
For by him all things were created, in heaven and on earth, visible and invisible, whether thrones or dominions or rulers or authorities—all things were created through him and for him.
And he is before all things, and in him all things hold together.
And he is the head of the body, the church. He is the beginning, the firstborn from the dead, that in everything he might be preeminent.
For in him all the fullness of God was pleased to dwell,
and through him to reconcile to himself all things, whether on earth or in heaven, making peace by the blood of his cross.
People take the idea of unity differently. Some people hear unity and they think it means everyone has to be the same. Other people hear “unity” and think it means they have to compromise all kinds of important things – give up their rights, show tolerance, etc.
You could say we are pretty divided on unity.
When people are not looking at themselves and how different they are, but instead look toward one similar goal, they come to unity. They might come with electric guitars or hymnals, they might come with expository Bible teaching or drama, but people that are moving towards the same goal inevitably come to some form of unity.
 making known to us the mystery of his will, according to his purpose, which he set forth in Christ  as a plan for the fullness of time, to unite all things in him, things in heaven and things on earth. (ESV)
How perfect is it, that God would not bring unity of all cultures to be mindless slaves, but that He would actually bring us, as Christians, into unity within Christ? A Pacific Islander can’t live the Christian life the same way as a new believer in St. Petersburg. (I’m talking about Санкт-Петербу́рг, not the one in Florida, but it works too.)
Uniting us in Christ means that as we follow Him more, and His life is expressed more in our lives, we all become more like Him together. It’s not that we’re becoming like each other except for the fact that we’re all changing to be like Him.
That is what united means. I don’t have to force you to be like me, and you don’t have to reject me because I’m not like you, but let’s encourage and draw one another toward Christ, in whom God is creating true and holy unity.