D. A. Carson, The Farewell Discourse and Final Prayer of Jesus: An Exposition of John 14–17 (Grand Rapids: Baker, 1980), 201–4:

To some people, the term ecumenism has only good connotations. Utter the word, and they hear harps playing and angels singing; or if harps and angels are deemed too ethereal, at very least a certain fire lights up their eye. To others the same word evokes only images of evil. Ecumenism is intrinsically a doctrine of compromise which emasculates the gospel and wickedly flirts with apostasy and assorted forms of unbelief. The first group tends to cite John 17 in its favor; the second group tends either to ignore John 17 or else to include within the unity only a very small group, while defining the unity in such innocuous terms (e.g., making it entirely a positional unity with no entailment for conduct) that it becomes difficult to see how such unity could ever serve as a witness of anything to the world. What does the text say?

The text refers to a unity of all true believers. The unity in question, as we have seen, is partly a function of being disciples of Jesus Christ, and partly something toward which we must grow and in which we must be perfected. This unity is not merely positional, for it is to function as a witness before the watching world; indeed, one might argue that it is the characteristic mark of the believing community.

On the other hand, we cannot help but observe that organizational unity is simply not in view. This is not to say that the unity for which Jesus prays could not issue in organizational oneness; but it is to say that organizational oneness is not fundamental or essential. The outward manifestation of this spiritual unity is not a neat organizational flow chart, but a compelling witness.

The problem is compounded today by the fact that Christendom uses the word Christian or the expression “disciples of Christ” in highly diverse ways. For some, a Christian is a person who believes in a supreme being vaguely associated with the Christian heritage. For others, a Christian is a moralist of sorts. Still others want the term to refer to everyone who has been baptized (by whatever mode and at whatever age) in the name of the Father, Son, and Holy Spirit. Yet [p. 202] again, others use the word to refer to those who claim to have been “born again” (whatever that means to them), or who have “accepted Christ as Savior” (which is not a New Testament category). How can “Christian unity” refer to something unambiguous when the term Christian means anything we want it to mean?

This is not the place to work out an essential set of beliefs and practices necessary to justify the appropriation of the word Christian. However, if the term is to have anything like its New Testament meaning, then for a person to be a Christian he cannot legitimately hold to a belief structure which the New Testament explicitly disallows, or adopt practices which the New Testament explicitly forbids. More positively, he must at very least hold to what the New Testament itself insists is a minimum confession or an essential practice. If he does not, he prostitutes the term Christian.

Let us come to cases. Suppose someone confesses, “Jesus is Lord” (cf. 1 Cor. 12:3). Does this guarantee that the person in question is a Christian? Regretfully, no. In the Corinthian situation, of course, the test of this confession was both a necessary and a sufficient criterion. There was very little dispute regarding who Jesus really was; and truly to confess this Jesus as Lord, in the midst of a polytheistic society where such an exclusivistic claim stood out, was unambiguously the work of the Holy Spirit. Today, however, the question of who Jesus is cannot be thought of as a given. To the classic liberal Jesus is the greatest moralist. To confess him as Lord means nothing more than to agree to follow high moral standards. To the Bultmannian, very little can be affirmed regarding the historical Jesus. To confess this Jesus as Lord is not to do more than open oneself to the possibility of authentic existence. For others, Jesus is both God and man, but his sacrifice on the cross was not a sufficient atonement for our sins: there must be additional sacrifice and penance. For such a person to confess “Jesus is Lord” may be to pass this confessional criterion with respect to Jesus’ person, while nevertheless falling under the curse of another criterion which lays down limiting beliefs concerning Jesus’ work (e.g., Gal. 1:8f.). Another person may confess “Jesus is Lord” to indicate his formal agreement with the tenets of orthodoxy (just as a person may recite superficially the apostles’ creed) yet fail to acknowledge Jesus’ lordship over his ethical practices, including both his private but consistently selfish conduct and his business practices. Such a person [p. 203] passes the test of 1 Corinthians 12:3 from a merely doctrinal perspective; but, failing to submit to the implications of lordship, he may fall under the searching tests of moral obedience and growing love (see, for instance, 1 John). In each of these instances, the confession “Jesus is Lord” serves as a necessary but not a sufficient criterion for being a genuine believer, a disciple of Christ, a Christian.

It does not take much knowledge of the current ecclesiastical and theological scene to recognize that, if basic New Testament tests are preserved and applied to the sweep of modern Christendom, not everything that calls itself Christian truly qualifies. Charitable as we may wish to be, that person is not Christian (in any New Testament sense) who steps unambiguously beyond the bounds of what the Scriptures recognize to be a true believer in and disciple of Jesus Christ—the Jesus Christ who has revealed himself in history and who is revealed in the pages of the holy Scriptures. This should not be thought surprising. After all, when Jesus came into the world and expounded the nature of God to his own race, only a remnant of his people perceived who he was. Judaism made many claims and reflected highly diverse belief structures; but not all Jews who espoused Judaism espoused Christ. Only a remnant came to faith in Christ. So, too, with Christendom: there are many belief structures and competing ethical and theological norms, but not everyone in “Christianity” (broadly conceived) espouses the Christ who has revealed himself to us. There will always be tares sown along with the wheat; and the final separation awaits God’s timing. Meanwhile we are very foolish or very naive if we think there is no difference between weed and wheat.

If these reflections have any validity, they may prove helpful when we try to assess the ecumenical movement. If ecumenists seek to join together into one organization all branches of “churchianity” known to Christendom, then they are trying to unite wheat and tares. Even if we acknowledge that every church has some true believers within it (and I am not certain that is so), the systematic denial of biblically required truth or the wholesale disregard for biblically mandated conduct in certain groups does not inspire confidence. John 17 does not look to a unity made up of both believers and members of the “world.” Anyone who is not a true believer constitutes part of that world which stands in antithetical relation to the unity of the church. Whoever cites John 17 to justify a unity that [p. 204] embraces believer and apostate, disciple and renegade, regenerate and unregenerate, abuses this passage. Such ecumenism has its roots not in Scripture but in misguided (if well-intentioned) notions of what New Testament Christianity is all about.

On the other hand, the things which tie together true believers are far more significant than the things which divide them. The divisive things are not necessarily unimportant: sometimes they are points of faith or practice which have long-range effects on the church for good or ill, reflecting perhaps some major inconsistency or misapprehension concerning the truth. Nevertheless the things which tie us together are of even more fundamental importance. Regardless of denominational affiliation, there ought to be among Christ’s people a sincere kinship, a mutual love, a common commitment, a deep desire to learn from one another and to come, if at all possible, to a shared understanding of the truth on any point. Such unity ought to be so transparent and compelling that others are attracted to it. To such biblical ecumensim (if I may so label it) there is no proper objection. Indeed, it is mandated by the Final Prayer of the Lord Jesus himself.

This book is available from Logos Bible Software in “The Select Works of D. A. Carson (7 vols.).”