That is a big question to ask, with every other religious sect claiming the label, and with enterprising entities claiming a lucrative niche market with products tagged as "Christian" this and "Christian" that.
J. Gresham Machen exhorts us to approach the question in a catholic frame of mind:
"But how shall we take a look at it? It has existed through some nineteen centuries and in a thousand different forms; how can we possibly obtain a common view of it, so as to include in our definition of it what it is and exclude from our definition what it is not? To what point in the long history of Christianity should we turn in order to discover what it really is? Surely the answer to that question is perfectly plain. If we are going to determine what any great movement is, surely we must turn to the beginnings of the movement. So it is with Christianity. We are not asserting at this point in our argument that the founders of the Christian movement had a right to legislate for all subsequent generations. That is a matter for further investigation. But what we are asserting now is that the founders of the Christian movement, whoever they were, did have an inalienable right to legislate for all those subsequent generations that should choose to bear the name 'Christian.' Conceivably we may change their program; but if we do change their program, let us use a new name. It is misleading to use the old name to designate a new thing. That is just a matter of common sense. If, therefore, we are going to tell what Christianity at bottom is, we must take a look at the beginnings of Christianity."
So here we see the foolishness of claiming Christianity while, in the same breath, neglecting what our spiritual forebears have left us as spiritual heritage. This is the modern malady:
"Christianity, according to that fashionable modern answer, is a life and not a doctrine, it is a life or an experience that has doctrine merely as its symbolic intellectual expression, so that while the life abides the doctrine must necessarily change from age to age."
If Christianity was merely concerned about living a life of moral excellence, then one need not be a Christian to achieve the same effect. One might as well be a devotee of any of the world religions in order to "be the best that I can be." Christian doctrine would have its place among Buddhist, Hindu, or Islamic doctrine, as a peer, as long as it can deliver the "goods."
But then Christian doctrine does not claim friendship with all other doctrines. In fact, it is hostile to them. It claims to be the only true account of reality as it is—and because it is true, it is life-changing!
"Was Christianity at the beginning in that sense a life as distinguished from a doctrine? At this point we desire to be perfectly clear. Christianity at the beginning certainly was a life, about that there can be no manner of doubt. The first Christians led lives very different from the lives of the people about them, and everything that did not conform to that peculiarly Christian type of life was rigidly excluded from the early Church. Let us be perfectly plain about that.
But how was that Christian type of life produced? There we come to the crux of the whole question. If one thing is clear to the historian it is that that type of life was not produced merely by exhortation or merely by the magic of personal contacts; if one thing is clear to the historian it is that earliest Christian missionaries did not go around the world saying. 'We have been living in contact with a wonderful person, Jesus; contact with Him has changed our lives; and we call upon you our hearers, without asking puzzling questions, without settling the meaning of His death, without asking whether He rose from the dead, simply to submit yourselves to the contagion of that wonderful personality.' That is, perhaps, what many modern men might have expected the first Christian missionaries to say, but to the historian it is clear that as a matter of fact they said nothing of the kind.
What they did say is summed up in a few words in the fifteenth chapter of the First Epistle to the Corinthians, where, as is admitted even by historians of the most skeptical kind, Paul is giving nothing less that a summary of what he 'received' from the very first disciples of Jesus in the primitive Jerusalem Church. 'Christ died for our sins according to the Scriptures – He was buried; He rose again the third day, according to the Scriptures' – there we have in brief compass what the first Christian missionaries said.
But what is that utterance that we have just quoted? Is it not an account of facts? 'Christ died, He was buried, He rose again' – that is a setting forth of things that happened; it is not an exhortation but a rehearsal of events, a piece of news.
The facts that are rehearsed are not, indeed, bare facts, but facts with the meaning of the facts. 'Christ died' is a fact; but to know merely that fact never did good to anyone; it never did anyone any good to know that a Jew, who was called Christ, died on a cross in the first century of our era. But it is not in that jejune [lifeless] way that the fact was rehearsed by the primitive Jerusalem Church; the primitive message was not merely that Christ died, but that Christ died for our sins. That tells not merely that Christ died, but why He died, what He accomplished when He died, but why He died, what He accomplished when He died, it gives not merely the fact but the meaning of the fact.
But when you say 'fact with the meaning of the fact' you have said 'doctrine.' We have already arrived, then, at the answer to our question. Christianity at the beginning, we have discovered, was not a life as distinguished from a doctrine or a life that had doctrine as its changing intellectual expression, but – just the other way around – it was a life founded upon a doctrine.
If that be so, if the Christian religion is founded upon historical facts, then there is something in the Christian message which can never possibly change. There is one good thing about facts – they stay put. If a thing really happened, the passage of years can never possibly make it into a thing that did not happen. If the body of Jesus really emerged from the tomb on the first Easter morning, then no possible advance of science can change the fact one whit. The advance of science may conceivably show that the alleged fact was never a fact at all; it may conceivably show that the earliest Christians were wrong when they said that Christ rose from the dead the third day. But to say that that statement of fact was true in the first century, but that because of the advance of science it is no longer true – that is to say what is plainly absurd. The Christian religion is founded squarely upon a message that sets forth facts; if that message is false, then the religion that is founded on it must of course be abandoned; but if it is true, then the Christian Church must still deliver the message faithfully as it did on the morning of the first Easter Day.
For our part, we adopt the latter alternative. But it is a mistake to think of us merely as 'conservatives'; it is a mistake to think of us as though we were holding desperately to something that is old merely because it is old and were inhospitable to what is new. As a matter of fact, we are looking not merely to a continuance of conditions that now prevail, but to a burst of new power. The Spirit of God will in God’s good time again enable men to see clear, and when they see clear they will be convinced that the Christian message is true. We long for the coming of that time. Now that the Christian message is so generally disbelieved or forgotten, the human race is sinking gradually into bondage; the advance in material things, extraordinary though it is, is being dearly purchased by a widespread loss of human freedom. But when the gospel is brought to light again, there will again be life and liberty for mankind." (emphasis mine)
Source: J. Gresham Machen, What is Christianity?