Is there a need for Christians to believe in Christianity at all, if they don't believe in Resurrection and Salvation through Christ alone? My question is directed to professing Christians who don't believe in any of the key doctrines held by evangelical churches, such as the Virgin Birth, the Resurrection, and Jesus Christ as the only path to salvation. If you believe in the evangelical doctrine of salvation, this question isn't for you.

I come from an evangelical background, which taught me to believe in the Virgin Birth, the Resurrection, and that Jesus is the only path to salvation. I understand that there are some Christians who don't believe in these things, which I think are the pillars and the cornerstone of Christianity, without which Christianity has no reason to exist. These Christians are commonly identified as "liberal Christians." You can read more about the view of this group of Christians here, and read a conservative theologian's response to their views here.

Back to my main question:

If one doesn't believe that Jesus is the only path to salvation, what is the "selling point" of Christianity? Why should people bother to become Christians at all? It would be equally okay to be an atheist, agnostic, or Buddhist who believes in Jesus as a great teacher and religion leader, but nothing more.   

By the same token, as I see it, if liberal Christians think that Bible is just another book, written and compiled by mere humans with human errors in it, then it is no longer infallible, and there is no reason to pay attention to Biblical teachings on any subject. So what is the difference between liberal Christians and non-Christians? Non-Christians can draw inspiration from the Bible in the same way as these Christians do. They believe in exactly the same thing!

I am curious: On what basis do liberal Christians identify themselves as Christians?
This is going to be a rather vague answer, mainly because even within the 'extreme liberal Christians' that you describe there is a huge range of belief. This answer is going to do no more than skim the surface of them.

In the UK, one of the best-known 'liberal Christians' is David Jenkins, former Bishop of Durham. Jenkins famously gave an interview, just before his consecration, in which he said that God would not have arranged a virgin birth, and that the resurrection was not a literal, physical event. This got him a lot of flak from traditional Christians, who believed he had abandoned the Christian faith. However in subsequent interviews it became clear that Jenkins was much closer to orthodoxy than first appeared. He did not deny the resurrection - he simply believed it was a spiritual event, rather than physical, and not any less real for that. He did not deny the existence of God, or that Jesus was his Son, or even the possibility of miracles. Jenkins has always described himself as Christian, and while his theology may be different from many, he certainly follows what he believes to be Christian truth. He is far from alone in these views.

At another point on the spectrum is Don Cupitt, who famously held the post of Dean of Emmanuel College Cambridge while denying the existence of God. ('Dean' is a religious post at Cambridge collleges, requiring one to be a priest of the Church of England). Cupitt's position is that modern knowledge has disproved the objective existence of God, and that Christian teaching is simply a recipe for a better way of living. He has stated that belief in God is good (even though God does not exist) because it leads to Christian practices. As Wikipedia puts it:


  Cupitt sometimes describes himself as Christian non-realist, by which he means that he follows certain spiritual practices and attempts to live by ethical standards traditionally associated with Christianity but without believing in the actual existence of the underlying metaphysical entities (such as "Christ" and "God")


In the US the best-known of the liberal wing is John Shelby Spong. Spong appears to fall into a similar camp as David Jenkins. While appearing to deny the literal physicality of the resurrection, he nonetheless affirms - extremely vehemently - the reality of it. He would deny the Bible's infallibility, but would affirm that through it you can discover the truth of a God of Love and a resurrected Jesus.

If there is a common thread here, it is that the people described here hold to what they believes is the underlying truth about God, Jesus and Christianity and for the most part are following the Jesus they believe in.