Why would an atheist be interested in this question? The answer is personal. Self-righteous hypocrites piss me off. I guess that is something I retain from youthful Sunday school lessons. Jesus inveighed against sanctimonious hypocrites more than any other group. “Whitewashed sepulchers,” he colorfully called them (Matthew 23:27), “…which indeed appear beautiful outward, but are within full of dead men’s bones and of all uncleanness.” When you hear someone piously condemn another as “not a real Christian,” you can bet your bottom dollar that what you are hearing is rhetoric that is at once both censorious and self-glorifying. Such rhetoric, if based on anything other than sheer bombast, rests upon demarcation criteria that are doctrinally tendentious and theologically question-begging.

Surely, though, there are some who cloak themselves in the mantle of Christian piety, but really are not Christians at all. After all did Jesus not also say “Not everyone that saith unto me Lord, Lord shall enter into the kingdom of heaven…(Matthew 7:21)?” Jesus goes on to imply that some of those who loudly profess their Christian credentials are in for a big letdown come Judgment Day. Doesn’t this imply that there are some who are false disciples? How, then, do you tell the false from the true? Must there not be some clear demarcation criteria after all?

First a word of caution from the philosophy of science: Philosophers long sought a demarcation criterion that would distinguish science from non-science. The most famous of these was Karl Popper’s doctrine of falsifiability. Popper argued that all genuinely scientific hypotheses are falsifiable. Thus, observation or experiment can overturn the claim of general relativity that the path of light curves in the vicinity of massive bodies. Ideological claims, on the other hand, are often presented in ways that, apparently, defy falsification. “God loves you and has a wonderful plan for your life,” for instance, seems quite compatible with God allowing you to perish painfully in infancy.

Popper’s criterion is intuitively appealing, but philosophically indefensible. The Quine-Duhem thesis is generally cited in response to Popper’s claim. That thesis notes that hypotheses do not face potentially falsifying evidence alone, but in the company of a host of auxiliary hypotheses. Lavoisier’s experimental demonstration that the products of combustion weighed more than the unburned material apparently constituted a straightforward refutation of the phlogiston hypothesis. Defenders of phlogiston, however, could claim that phlogiston had negative weight, so its escape in the process of combustion increased the weight of the combustion products! Of course, this auxiliary hypothesis is totally ad hoc, but it demonstrates that potentially falsifying evidence might be accommodated by ruling out an auxiliary assumption, e.g. that phlogiston would have positive weight, rather than falsifying the phlogiston hypothesis itself. Falsification is therefore not the straightforward sort of thing Popper thought that it was.

A simpler critique of the project of developing demarcation criteria is to note that such criteria seem incapable of walking the razor-fine line between excessive stringency and over-laxness. If demarcation criteria are made too strict, they will rule out unquestionably scientific enterprises, like, say, geology or astrophysics. If they are too loose, they will admit flapdoodle like Young-Earth Creationism into the scientific fold. Likewise, anyone wanting to formulate criteria for being a true Christian would need to avoid criteria that ruled out St. Francis or ruled in Bertrand Russell.

The upshot is this: If demarcation criteria to distinguish science from non-science are perennially elusive, then, prima facie, would not the distinction between Christian and non-Christian be expected to be similarly problematic?

So, then, how would we proceed to spell out necessary and/or sufficient conditions for being a true Christian? Maybe we should check some examples. A quick Internet search yielded the following definition:

A true Christian is a person who has put faith and trust in the person and work of Jesus Christ, including His death on the cross as payment for sins and His resurrection on the third day. John 1:12 tells us, “Yet to all who received him, to those who believed in his name, he gave the right to become children of God.” The mark of a true Christian is love for others and obedience to God’s Word (I John 2: 4, 10). A true Christian is indeed a child of God a part of God’s true family, and one who has been given a new life in Jesus Christ. (http://www.gotquestions.org/what-is-a-Christian.html).

Though admirably succinct, this is hopelessly vague. What does it mean to “…put faith and trust in the person and work of Jesus Christ…?” The sentence goes on to mention the belief that Jesus’s death on the cross paid for sins and that he was resurrected on the third day. Does the “payment for sins” wording imply that having faith in the person and work of Christ requires adherence to the substitutionary theory of atonement? Is this the only one allowable, with advocates of all other soteriologial theories destined for hellfire? Must a Christian believe in the bodily resurrection of Jesus? Would this mean that, say, Rudolf Bultmann was not a Christian because he did not accept the Resurrection as an objective historical fact? So, a true Christian loves others? All others? OK, so if I encounter someone acting or speaking in a hateful way towards, say, liberals, feminists, environmentalists, atheists, evolutionists, GLBT persons, Muslims, or Democrats, may I conclude that this person is not a Christian? How do we distinguish one who is not a Christian from one who is merely a hypocrite?

Further, what is obedience to God’s Word? Might there not reasonably be considerable disagreement as to the content of God’s Word and what constitutes obedience to that Word?

Here is another, somewhat longer set of criteria offered, I think, as necessary conditions by Roger E. Olson:

1) A Christian will confess Jesus Christ as God and (the only) savior…

2) A Christian will be baptized into the body of Christ or at least have the baptism of desire (as the Catholic church puts it).

3) A Christian will, of course, believe in the triune God, Father, Son and Holy Spirit, even if he or she is confused about the doctrine of the Trinity. (…I cannot consider someone who entirely rejects God’s triunity a Christian.)

4) A Christian will believe in a supernatural worldview–that is, nature is not all that is real. Whether they believe in miracles today is not the issue; the criterion is that a real Christian will believe that God is not just a dimension of the universe or of human existence (e.g., the “call to self-transcendence”).

5) And, of course, a Christian will regard the Bible as the Word of God, sufficient and normative for Christian belief and practice https://www.patheos.com/blogs/rogereolson/2011/11/who-is-a-christian-some-suggested-criteria/.

This is better but still deeply problematic. The first criterion is again quite vague since there have historically been many different views on salvation and Christ’s role as savior. Olson is willing to admit that a “baptism of desire” is sufficient for inclusion in the Christian fold, but I note that others on other sites require water baptism. Further, by these criteria, Unitarians cannot be Christians. For instance, the fanatically pious Isaac Newton could not have been a Christian because he did not accept the Trinity. Finally, it is not at all clear what it means to “…regard the Bible as the Word of God.” Avowed Christians have many different views of Biblical authority. Some are inerrantists (“The Bible says it; I believe it; that settles it”), some are infallibilists, and some regard many passages as plainly mythological or ideological. The Church Father Origen interpreted scripture allegorically. Martin Luther called the Epistle of James, an “epistle of straw” because he held that it puts too much emphasis on good works rather than grace. Even St. Augustine held that a scriptural passage should be set aside if its claims are clearly contradicted by science. In general, as I understand the Catholic view, the authority of Scripture must be balanced by reason and Church tradition. Clearly, then, there has been considerable variation in the interpretations of the authority of scripture. Which view is the correct one and why?

The “why” is the most important question with respect to all purported criteria. Olson modestly says that he is merely “suggesting” these. However, if any such set of criteria is to count as more than a suggestion, it needs to be backed by some authority. Unfortunately, Scripture contains no checklist of criteria. There are many passages that prescribe or proscribe certain beliefs and practices (after all, religion is in the prescribing and proscribing business). Yet, it is not clear which doctrine and praxis are essential, that is, which ones are such that one is just not a Christian if one does not believe those things and do those things. 1 John 2:4 says, “He who says ‘I know him’ but disobeys his commandments is a liar and the truth is not in him.” Which commandments are essential for Christians to obey if they are to be counted as Christians? All of them? In that case, things are simple: There are no Christians at all. Besides, as noted above, the nature of God’s commands and what it means to obey them have been items of vigorous debate throughout the history of Christianity—right back to Paul’s disputes with Peter.

Perhaps, then, given the multifarious and intractable difficulties involving the formulation of clear, rigorous, and authoritative demarcation criteria, some would take a different approach. Perhaps the way to distinguish Christians from non-Christians is to take a paradigm-case approach. Really, outside of disciplines such as mathematics and formal logic, completely adequate definitions are hard to come by. However, we have no problems distinguishing ravens from writing desks even though we lack precise definitions of those items. We make everyday distinctions on the basis of paradigm cases. Twilight means that no clear boundary between day and night can be drawn, but we all know that it is day at noon and night at midnight. Maybe “true Christian” is something we cannot define, but we can all recognize one when we see him or her. Perhaps the way to proceed is to offer lists of Christian paragons and judge dubious cases by how like or unlike they are compared to those paragons.

The problems here is that one person’s list of paragons might hardly overlap with another’s. If we made a list of some of the most prominent avowed Christians of history, we would find that many regarded others on that list as heretics or apostates. In the end, probably only the Apostle Paul would be on everyone’s list. But Paul said that slaves should obey their masters (Colossians 3:22) and that women should shut up in church and wait until they get home to ask their husbands if they have any questions (I Corinthians, 14: 34-35). Are such sentiments required of all Christians? Of course, Christians have tried, desperately, to explain away these odious sayings of Paul’s—not too successfully, in my book.

OK, well why not just make Christ himself the paragon? As the medieval devotional adjured, practice the imitation of Christ. Someone is a Christian if he or she is Christ-like. Surely, though, a mere human cannot come anywhere close to the perfection of the Son of God, so being Christ-like must mean some degree of approximation of Christ’s goodness. However, the question of criteria will inevitably and insidiously creep back in for those who want to make Christ-likeness the standard for being a true Christian. How closely must one approximate the behavior of Christ to be a Christian? Again, if the standard is set too high, we will have to conclude that there are no Christians. If the standards are looser, might we not have to admit Mahatma Gandhi into the Christian fold since, by many measures, his behavior was much more Christ-like than that of many avowed Christians? In general, would not the one—though he is, say, a professed Hindu—who practices forgiveness, humility, charity, and meekness be more Christ-like than one who affirms every point of the Nicaean creed, but is a vindictive, egotistical, uncharitable, and arrogant jerk? Hmmm. Maybe that is what Jesus meant when he said “Not all who cry unto me “lord, lord…”

In conclusion, one who wants to condemn other avowed Christians as fakes has a heavy burden of proof: He must defend some set of demarcation criteria as authoritative while not begging theological questions or making partisan assumptions or offering tendentious, self-serving interpretations of scripture. Otherwise his proposed criteria will amount to nothing other than an exercise in sectarian (and self) glorification.