What binds us together?
Church unity has these aspects; Christians are united because they...
Some Christian groups insist that their denomination or Church is the one true Church and that non-members are outside the Church (and are probably not really saved). The Catholic Church claims to be the one true Church but allows that Protestants are actually Catholic — they just don't realize it! Many denominations and non-denominational denominations disparage what they call "religion" (not realizing that true Christianity is itself a religion).
Christ's church is united — it is not fractured. Faith-filled, believing Christians are in unity with one another and with Christ. People who think otherwise have a wrong view of the nature of the Church.
Putting all the pettiness, ignorance, and small-mindedness aside, every person who ends up redeemed is united with every other. So why are we so ill-mannered, boorish, and rude to our fellow Christians? We should stop.
Unity is of the heart, not institutional, as these New Testament passages make clear...
My prayer is not for them alone. I pray also for those who will believe in me through their message, that all of them may be one, Father, just as you are in me and I am in you. May they also be in us so that the world may believe that you have sent me. I have given them the glory that you gave me, that they may be one as we are one: I in them and you in me. May they be brought to complete unity to let the world know that you sent me and have loved them even as you have loved me. (John 17:20-23)
They are in unity because they are united to Jesus, not because they have an external institutional unity. Once a person becomes a member of the body of Christ they are by definition in unity. God the Father certainly answered Jesus' prayer, so this unity is real not merely an elusive goal.
Christians are to have a spirit of unity. This does not mean an institutional unity but that we love one another.
Make every effort to keep the unity of the Spirit through the bond of peace. There is one body and one Spirit, just as you were called to one hope when you were called; one Lord, one faith, one baptism; one God and Father of all, who is over all and through all and in all. (Ephesians 4:3-6)
By living at peace with one another, Christians are to manifest the unity they already possess in Christ.
It was he who gave some to be apostles, some to be prophets, some to be evangelists, and some to be pastors and teachers, to prepare God's people for works of service, so that the body of Christ may be built up until we all reach unity in the faith and in the knowledge of the Son of God and become mature, attaining to the whole measure of the fullness of Christ. (Ephesians 4:11-13)
The unity referred to here is not an institutional unity.
Bear with each other and forgive whatever grievances you may have against one another. Forgive as the Lord forgave you. And over all these virtues put on love, which binds them all together in perfect unity. (Colossians 3:13,14)
Having love for one another is the essence of the unity of the faith.
In the context, Paul is clearly not referring to an institutional unity but to unity of the faith, of belief, of a brotherhood of love.
The unity within the Trinity is not institutional but a unity of essence and purpose.
Paul allows for various views regarding whether or not a person will eat food sacrificed to idols. Thus, he does not demand a fixed rule for all Christians on non-essential issues.
Christ's church is united — it is not fractured. Faith-filled, believing Christians are in unity with one another and with Christ. What is often missing is that Christians don't relate to one another in love.
Christians are therefore free to establish a multitude of religious institutions — many churches, denominations, and communities. These are all in unity with one another if we would just believe it and act like it.