By Tim Harmon
What criteria should we use to evaluate who is a Christian, and who is not? Does it matter if someone affirms a particular set of doctrinal points? Must he or she follow a specific moral code?
Since the advent of the modern missions movement, such questions have been particularly pressing. In contexts with significant language and culture barriers, it can be difficult to assess the validity of a person’s professed conversion.
One solution to this problem was set forth in the 1970’s by Paul G. Hiebert. And now, some 40 years later, his insights into the matter are impacting how western Christians think not just about the mission field – but also their home turf.
Then and There
Hiebert, a missiologist and cultural anthropologist, questioned how much of the gospel message an illiterate and impoverished person in a non-western context must understand in order to be a Christian. 
What if a person could not read the Bible, knew no other Christians, and had only heard the gospel once. Could such a person become a Christian? Surely God knows the heart. But how do we, this side of heaven, discern who is a Christian and who is not? What criteria can we use?
Using set theory (a type of mathematics), Hiebert proposed that this matter could be evaluated quite differently, depending on whether one subscribes to a “bounded set” or a “centered set” model.
The basic idea of each model is shown below.
Bounded Set: This type of set is static – it’s all about boundaries. Christians are those people who affirm right beliefs, and practice right behaviors. Those who don’t aren’t.
Centered Set: This type of set is dynamic – it’s all about movement in a particular direction. Christians are those moving toward Jesus, rather than away from Him.
Hiebert pointed out that from a bounded set perspective, a person like the one described above may not be able to be identified as a Christian. Such a person may not have a solid grasp of orthodox theology, and may not even understand basic Christian moral imperatives.
However, using a centered set model, if movement had been made toward Christ, such a person could be identified as a Christian.
But what does all of this have to do with those of us who live in a literate, affluent western context?
Here and Now
In recent years, the “centered set” paradigm has been adopted by a number of western Christian thinkers – particularly those in post-conservative/post-foundational circles. In their view, much of what is in need of reform in the western church can be ascribed to its adherence to bounded sets.
For some in this camp, the discussion has gone beyond the topic of conversion, to include matters of ecclesiology and orthodoxy.
Concerning the church, items that serve to promote barriers, such as church membership, doctrinal statements, and moral/ethical standards are often seen as doing more harm than good. Instead, the church is to be a place where people of all backgrounds can belong (even before they believe), freely gathering, and working together in their movement toward Christ.
Regarding orthodoxy, centered-set advocates are not so comfortable with detailed, dogmatic doctrinal statements, especially when it comes to secondary matters. They want to keep the main thing the main thing – and that main thing is Jesus. As long as people are moving toward him, all other matters are secondary.
In a sense, it is fitting that a model initially applied to pre-Christian cultures is now gaining traction in a post-Christian one. And certainly, there is much to admire about centered set thinking. Within the church, we do not want to erect illegitimate barriers to belonging. No doubt, in the past the church has done just that.
Still, centered sets are inadequate on their own. To assert that the goal is movement toward Jesus or the gospel is essentially meaningless without first defining “which Jesus?” and “what gospel?” And once we start defining terms, we’re about the business of boundary construction.
The truth of the matter is this: all centered sets have boundaries, and all bounded sets have a center. The two models are not antithetical, but rather differ in terms of emphasis.
All in all, the temptation to make these categories mutually exclusive ought to be avoided. At one extreme, an overly centered set will attempt to herd opposing beliefs under one umbrella, resulting in confusion. At the other, a too tightly bounded set will tend to promote rigid exclusivity, ignoring the all-important center.
Considering the benefits and pitfalls of either method at their extreme, both may be applied positively by way of emphasis. All Christians can agree that their faith ought to be Christocentric, and few deny that some orthodox boundaries are necessary.
Let’s not fall into the trap of trading one error (overly rigid boundaries) for another (no boundaries at all). Rather, let us keep Jesus at the center, while recognizing that God’s people are called to be discerning (1 Jn. 4:1-6), distinct from the world (1 Cor. 5:9-12; 1 Jn. 2:15-17), and doctrinally precise (1 Tim. 6:20).