Some years ago I attended the early morning Communion service in Magdalene College Chapel, Cambridge. The service was rich in Trinitarian worship, in elevated God-centered prayers, in excellent hymns, and in a very fine, if too-brief sermon. As I sat and shared in the worship, I thought, “How diverse the church of Jesus Christ is.” Here I was in a high Anglican College Chapel, a head-to-toe Westminster Calvinist and a convinced Presbyterian. How do we consider the unity of the church in the midst of such differences?
The Westminster Confession of Faith has a magnificent chapter entitled, “Of the Communion of the Saints”: 

All saints, that are united to Jesus Christ their Head, by His Spirit, and by faith, have fellowship with Him in His grace, sufferings, death, resurrection, and glory: and, being united to one another in love, they have communion in each other’s gifts and graces, and are obliged to the performance of such duties, public and private, as do conduce to their mutual good, both in the inward and outward man... Which communion, as God offers opportunity, is to be extended unto all those who, in every place, call upon the name of the Lord Jesus (WCF 26).

It is only too easy for Christians to exclude practically, and even principially, from the communion of the saints people in churches and denominations who do not believe what they believe. There are, of course, foundational and non-negotiable gospel doctrines. Paul writes in 1 Cor.15:1-3 of things that are “of first importance.” There is a core of truths that belong to the essence of the Christian faith. If people deny any of these core truths, they exclude themselves from the communion of the saints. But if they believe these truths and trust the Saviour embedded in these truths, they are “family” and must be acknowledged and embraced as such. They may have defective views of divine sovereignty, not understand the priority of grace over faith, have uncertain views of Genesis 1, have fantastical views on the end times, and promote the continuation of charismatic gifts; but if they hold to Christ the Head, they are family. If the Father has elected them, the Son has shed his blood for them and they are indwelled by the Holy Spirit, they are family. 
Family life can be problematic and, sadly, can be dysfunctional. But family is family no matter how problematic and dysfunctional.
It is a huge and searching test of our gospel commitment that we recognize that the Lord has his people everywhere, often in what may appear to us the strangest of places. Recognizing, accepting and embracing brothers and sisters in Christ, however different they are from us, was something Jesus strongly impressed on his disciples. Mark 9:38–41 is one of the most challenging episodes recorded in the Gospels. The disciples saw a man casting out demons in Jesus' name, but they told him to stop, “because he was not following us.” Jesus immediately rebuked his narrow-hearted disciples, “Do not stop him.” The disciples could not see beyond themselves. If this man was not one of them, he could hardly be one of Christ's! How sad. The Savior strongly rebuked them.
The moment we identify the Christian faith with our particular church or denomination or confession of faith, that moment we become a sect and not a true church of Jesus Christ.

None of this means that we are never to try and correct our brothers and sisters and lead them into a richer, more biblical understanding of the faith. It does mean that our default is not, “They need putting right,” but, “These are my family, we are fighting under the same banner, we are heading for the same heavenly country, we are washed in the same blood.” In a justly famous letter to Archbishop Thomas Cranmer, John Calvin wrote, 

This other thing also is to be ranked among the chief evils of our time, viz., that the churches are so divided, that human fellowship is scarcely now in any repute among us, far less that Christian intercourse which all make a profession of, but few sincerely practice…. Thus it is that the members of the Church being severed, the body lies bleeding. So much does this concern me, that, could I be of any service, I would not grudge to cross even ten seas, if need were, on account of it. 

Sometimes, family disagreements become public and there may be a good reason why. Most of the time, family disagreements remain within the family; not just because it is never good “to wash your dirty linen in public,” but more importantly, because God's blood-bought children should always want to please their heavenly Father and the Saviour to whom they are united by the Holy Spirit. Like all fathers, our heavenly Father desires to see his children live in harmony.

I often wonder how seriously we take to heart Jesus' prayer to his Father that his people be one (John 17). As the consuming darkness of the cross was beginning to invade his human soul, our Lord prayed, “The glory that you have given me I have given to them, that they may be one even as we are one, I in them and you in me, that they may become perfectly one, so that the world may know that you sent me and loved them even as you loved me” (John 17:22-23). So that the world may know!

Our Confession's chapter on The Communion of the Saints needs not only to be believed and confessed, it needs to be practiced. How that will be done will demand humility, courage, love, and resolve.