True Christian unity can be achieved only by the truth of God’s Word, within the boundaries of gospel essentials, and centered in the whole council of God. Minimization of any of God’s truth inevitably leads to the erosion of doctrine and the ultimate dissolution of true Christian unity.

God clearly wants Christians to be unified, and Christians should pursue unity wherever possible. Jesus expressed this in his high priestly prayer of John 17. He said in verse 21, “That they may all be one, just as you, Father, are in me, and I in you, that they also may be in us, so that the world may believe that you have sent me.”

Yet many evangelicals today champion a particular idea of Christian unity that has two problematic results: (1) lack of separation from the world, and (2) a minimization of important biblical doctrines that are deemed not to be “essential” to the gospel.

This is why it is important to recognize the nature of the unity for which Christ is praying. Thankfully, Christ’s own statements in his prayer clarify what Christian unity will look like. In verse 14, he says,

I have given them your Word, and the world has hated them because they are not of the world, just as I am not of the world.

Whatever this unity is—this unity that will make Christ known to the world—it is a unity that separates us from the world. Christ says that our beliefs in his Word will actually cause the world to hate us; our unity with him and his Word will set us apart from the world. Christ says that believers are sanctified—“set apart”—from the world by the truth of his Word:

Sanctify them in the truth; your Word is truth. As you sent me in the world, so I have sent them into the world. And for their sake I consecrate myself, that they also may be sanctified in truth. (Jn 17:17)

Christian unity is not, as many practice today, a minimization of doctrine so that we can all get along and reach the world. On the contrary, our unity that will reach the world is based on being distinct from the world and set apart by the truth of the Word.

In any discussion of Christian unity we must remember this: unity is possible only around truth, and unity around truth will make the world hate us.

Boundary and Center

What this means, then, is that there is a boundary around Christian unity. There can be no unity with those who do not believe the gospel; Christian fellowship is impossible with those who deny the fundamentals of the gospel, including the inerrancy of Scripture, the virgin birth, the deity of Christ, substitutionary atonement, and justification by grace alone through faith alone in Christ alone. This is what John emphasized in his second epistle, when he wrote, “If anyone comes to you and does not bring this teaching, do not receive him into your house or give him any greeting, for whoever greets him takes part in his wicked works” (2 Jn 1:10). The gospel is the boundary of Christian unity.

Over the last fifteen years or so, conservative evangelicals have talked a lot about the gospel as the center of Christian unity—it is what bring us together; it is what we unite around. All other doctrinal issues should be set aside, they say, in order for us to be unified around what is really important—the gospel.

But this thinking actually has it backward. Contrary to these popular evangelical movements, the gospel is not the center of Christian unity; the gospel is the boundary of Christian unity. The gospel does unify believers, but it does so in that it separates us from those who do not believe the gospel.

The center of Christian unity is the truth of God’s Word—all of it. The gospel is the boundary of Christian unity, but the center of Christian unity is the whole counsel of God, all of the truth contained in his inspired, inerrant, authoritative, and sufficient Word.

All of God’s truth matters; all of God’s truth affects Christian unity to one degree or another. The Christian faith is more than just the gospel—it is the whole council of God. Doctrinal matters beyond the fundamentals of the gospel like baptism, ecclesiology, hermeneutics, eschatology, and so much more are secondary to the gospel—they’re not the boundary—but they are important and affect the degree to which we can unify and cooperate with other Christians.

For example, I might be able to stand side-by-side with a conservative Presbyterian in order to preach the gospel—we are both unified at a certain level by our common belief in the fundamentals of the gospel, but I would not be able to plant a church with him given our disagreement regarding church polity and baptism. I may be able to teach in an educational institution where there exists considerable diversity in a number of areas that I believe to be extremely important, but I would not be able to join a church with the same spread of divergent views.

In other words, Christian unity necessarily has two levels: unity within the boundary of the gospel, and unity centered on other important biblical doctrines and practices. The more agreement I have with someone in these other matters, the more unity I can have with him. Conversely, just because I might affirm that someone is a Christian who is inside the boundary of the gospel does not mean that I will be able to unify with him on every level.

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