What is there in Christianity as a true, clear criterion to distinguish between truth and falsehood? Especially since we know that there are many, many denominations.
positions do not make a person Christian... valid baptism makes a person Christian... Woodrow for example is considered a (lapsed) Christian while Qatada would not be considered a Christian.
after a person is baptised they still may not be considered members of the Church (that is called excommunication) or their theology might be considered unorthodox (that is called heresy). but they are still considered Christian by virtue of baptism.
martin luther was a Christian but he was excommunicated and heretical... as was arius. born protestants or orthodox are not considered excommunicated because they were not brought into the Church to begin with... but the errors of their beliefs make the heretical to varying degrees.
the issues of baptism, heresy and excommunication all affect our standing on judgment day.
As I said, we obviously argue amongst ourselves. So, for a person outside of Christendom it might be hard to say. And even for those within it, there are disagreements.
By Jayda's definition there are some folks connected to my church that were presented for baptism as infants by their parents 50 years ago and who have not been back in the church since that would be considered Christians. Yet there are others -- children presently active and involved in the life of the congregation, who believe in the historic teachings of the Christian faith, who have made personal confessions of their own faith in Christ, and regularly participate in receiving Holy Communion, but because their parents made the decision not to have them baptized until the child made this commitment for themselves -- who would not be considered Christian. So, as some of these children have approached me to schedule baptism, I suppose that on that date, though their theology won't have changed, their status in the eyes of the Catholic Church will.
Other Christians would cite making a personal confession of faith as the key to their evaluation of whether or not a person is a Christian. Such individiuals hold that if someone walks down the sawdust trail, responds to an altar call, prays a prayer asking Jesus into their heart that this person is a Christian at that point in time. For these individuals it matters not whether the person has participated in the life of the church before or not. Some people don't even seem to be concerned about whether such a person participates in the life of the church after making such a profession of faith. Though generally most who make such a commitment seek to be baptized if they have not previously been baptized, the baptism itself is not seen as the moment of becoming a Christian, rather it was when the person "accepted Christ as my personal savior" that is understood as the defining moment.
And then still other Christians look to other events as what they use as the key deciding moment. So, since we disagree amongst ourselves, it is no doubt hard for someone outside of Christianity to sense that there is any consistent oneness in the body of Christ. Yet, surprisingly, there really is. Notice that we still have no problem, even in our disagreements, with using the term Christian to describe theologies, positions, or people. We sometimes hold other people to be in error in the specifics of their definitions, but there is enough common understanding that we don't debate one another over the general use of the term. Even non-Christians seem to understand this.
So, what does this mean? For me it means that while at the edges it is hard to draw an exact line and say that this person is Christian and this person is not, and get universal agreement. That it is very easy to recognize what is at the center of Christianity. I suggest that this center is what should be the standard by which you answer your question as to what is used to distinguish truth from falsehood, or Christian from non-Christian teachings. It is sort of like asking the question "What is normal?". Is it normal to think of a family as a single mom raising children of two different fathers while living with a third man? I hope not. But I, also, don't think anyone would view it as abnormal to encounter such a "family". Yet, even if it is not abnormal, that is not what anyone would want to use as the standard by which one identified what is a family. So it is with Christianity. There are things that pass for Christian that I would not want to have people use as the standard to identify what is Christianity.
For me, the standard remains that at the heart of Christianity is belief in Jesus Christ as God incarnate, come to live among us and make God known to us, creating a new covenant by which lost human beings can be reconnected to God by grace (not a reward for something we have done), and we trust in Jesus Christ to accomplish this for us by his sacrificial and atoning death on the cross.