Since most churches are formally independent, it can be difficult for various bodies of believers to think cooperatively about the biblical theme of unity between churches. Yet, Jesus wanted his disciples to be one even as he and the Father were one (John 17). There are numerous biblical passages that talk about relationships between churches. For example, they all shared:
- Love and greetings (Rom. 16:16; 1 Cor. 16:9; 2 Cor. 13:13; Eph. 1:15, 4:22; Col. 1:4)
- Preachers and missionaries (2 Cor. 8:18, 3 John 1:5–6)
- Financial support for one another (Acts 15:25–26; 2 Cor. 8:1–2, 8:9–12)
- They also imitated one another in Christian living (1 Thess. 1:7, 2:14)
The local congregations of the New Testament were very much integrated with one another. And their interdependence was grounded in the fact that they shared three realities in common that encouraged cooperation: the same Christ, the same confession, and the same commission.
Reality No. 1: We Share the Same Christ
Christians and churches share the same Lord and Christ. Notice Paul’s greeting to the church in Corinth: “To the church of God that is in Corinth . . . called to be saints together with all those who in every place call upon the name of our Lord Jesus Christ, both their Lord and ours” (1 Cor. 1:2, 2 Cor. 1:1).
Christians and churches share the same Lord and Christ.
Churches may be self-governing, but they are all made up of “fellow citizens . . . members of the household of God” (Eph. 2:19). They all belong to the same government—the kingdom of Christ―and so are politically united even though geographically separate. For example, American citizens who work at the US embassies in London and Paris share a common bond, even if they don’t possess any direct authority over each other.
Practical Implications of Sharing the Same Christ
The reputations of all Christians in all churches are bound together, regardless of denomination. When one church presents a poor witness in a city, every church in that city suffers. When one church presents a positive witness, every church benefits. Churches, therefore, share an interest in one another’s spiritual welfare.
- Mutual Aid
Since churches share an interest in one another’s spiritual welfare, they should pray for one another, encourage one another, financially support one another as opportunity allows, and generally do what they can to support one another’s ministries. This means there should be an openness to informal relationships among churches, particularly among church leaders. Knowledgeable relationships facilitate specific prayer, encouragement, aid, and help guard against “turf-wars.”
Different levels of cooperation are possible based on different levels of doctrinal and ecclesial unity. Two Baptist churches can work together to share the gospel and plant churches because they share the same gospel and the same polity. A Baptist and a Presbyterian church can work together to share the gospel but not to plant churches because—though they share the same gospel—they do not share the same polity.
Reality No. 2: We Share the Same Confession
Different Christian churches share the same gospel confession, even when they belong to different denominations. Think of how Paul exhorted the churches of Galatia: “If anyone is preaching to you a gospel contrary to the one you received, let him be accursed” (Gal. 1:2, 9).
Practical Implications of Sharing the Same Confession
Churches should partner in learning from one another and teaching one another. The earliest churches did this in the sharing of preachers and guest missionaries. Today, this can be done in attending or hosting conferences, supporting seminaries and the seminary educations of aspiring pastors, subsidizing healthy Christian publishers, and starting local ministerial associations.
Churches should learn from other churches in different eras. The practices and patterns of pastors in the past (such as how ministerial associations played a critical in prompting the Baptist missions movement in the 1800s) should inform how we address challenges in the present. 
Churches should imitate one another in holiness, just as the apostolic churches imitated one another (1 Thess. 1:7, 2:4; 2 Thess. 1:4). Paul sought to “remind” the Corinthian church of his “ways in Christ,” as he taught “them everywhere in every church” (1 Cor. 4:7). This reality points to the value of multichurch conferences, books, and ministerial associations. But it particularly highlights the need for pastors to build relationships with one another beyond their own churches, as they seek to grow both in personal holiness and pastoral wisdom.
Churches should work to supply capable pastors or at least supply preachers to struggling churches that lack them, as a gift from one sister to another.
Reality No. 3: We Share the Same Commission
Jesus commissioned all churches to make disciples (Matt. 28:18–19). And they are each tasked with guarding the name and reputation of Christ through church discipline (Matt. 18:15–20).
Practical Implications of Sharing the Same Commission
- Church Membership
Churches should help one another with membership and discipline. I don’t believe one church can exercise authority over another, but I’ve watched our church work well with other congregations in the transfer of members, as well as in the exercise of discipline. Working together in matters of membership and discipline helps us make and oversee Christ’s disciples and so fulfill the Great Commission.
- Evangelism and Missions
Churches should work together in missions and evangelism. This can happen locally, as when a church partners with nearby churches to lead evangelistic Bible studies at lunchtime in the business district. Or it can happen nationally and globally, as when the churches of the Southern Baptist Convention pool their money to train and send missionaries overseas.
- Mercy Ministry
Churches should partner together in their mercy ministry work. Paul provided the most obvious biblical example when he collected support from multiple churches for the church in Jerusalem. Churches today do well to look for ways to support sister churches with fewer resources at their disposal. Such support helps Christ’s kingdom and serves the Great Commission.
Biblically faithful churches recognize their interdependence with other churches.
Not only does a biblically faithful church understand its independence, it also recognizes its interdependence with other churches. These three realities—the same Christ, the same confession, and the same commission—provide the biblical basis for healthy interdependence among churches who affirm those realities.
When churches are interdependent, they work together to fulfill the Great Commission.  They pray for, encourage, challenge, and support one another, knowing that the success of one is the success of all, and the defeat of one is the sorrow of all (1 Cor. 12:26).
Jonathan Leeman is an elder at Capitol Hill Baptist Church in Washington, D.C., editorial director of 9Marks, and author of several books, including Church Discipline: How the Church Protects the Name of Jesus and Don’t Fire Your Church Members: The Case for Congregationalism. You can find him on Twitter at @JonathanDLeeman.
This article is an adaptation from Don’t Fire Your Church Members: The Case for Congregationalism, (posted here by permission from Broadman & Holman).
 See the two separate 9Marks Journals on how churches can care for one another: http://www.9marks.org/journal/church-and-churches and http://www.9marks.org/journal/wanted-apostolic-pastors.
 See Michael Haykin, “Baptists Reading Together and the Birth of Modern Missions,” http://www.9marks.org/journal/baptists-reading-together-and-birth-modern-missions.
 See Bobby Jamieson, “The Great Commission is Bigger Than Your Church,” http://www.9marks.org/journal/great-commission-bigger-your-church.