Is there a need for Christians to believe in Christianity at all, if they don't believe in Resurrection and Salvation through Christ alone?

I'm not entirely sure whether or not my denomination, the Holy Roman Catholic Church, counts as 'extremely Liberal' according to your definitions. However, together with the (also possibly 'extremely liberal') Holy Orthodox Churches, we teach that there is a lot more to religion than just salvation, and these teachings are certainly possibly held by people holding all of the 'liberal' tags you describe.

Christianity is, for us, a way of life. We live it perhaps at first out of fear of God's wrath or desire for God's gift of salvation, but ultimately we teach that the greatest Christians serve God out of love for Him.

Example prayer showing doctrine in practice:

Prayer of Love for the Crucified Lord

O Jesus, it is not the heavenly reward You have promised which impels me to love You; neither is it the threat of hell that keep me from offending You. It is You, O Lord, it is the sight of You affixed to the Cross and suffering insults; it is the sight of Your broken body, as well as Your pains and Your death. There is nothing You can give me to make me love You. For even if there were no heaven and no hell I would still love You as I do.
Amen.

Christianity is right. God is all-good, and deserving of all of our love. God asks us to be Christian. Does it really matter why? To reject God is wrong, indeed it is the most wrong it is possible to be. As Kierkegaard correctly proposes, it is right to have faith in God no matter what. That means even without salvation, the resurrection, logical consistency, or anything else.

In the story of Isaac and Abraham, it is precisely because what God asks of Abraham Abraham would ordinarily believe to be morally wrong that Abraham's obedience is indicative of great faith-- Abraham trusted God's righteousness over his own understanding of morality-- over reason, knowing that God would never ask him to do something that was truly unethical. In teleologically suspending the ethical, Abraham shows his great faith to God, who then prevents the sacrifice from occurring (probably since it would, in fact, have been evil). Such faith is laudable, and the resurrection-- just like God's sparing of Isaac-- is only incidentally related to it.

Just because you don't believe in the resurrection does NOT make it "equally OK to be a(n) atheist/agnostic/Buddhist who believes in Jesus as a great teacher and religion leader but not more", at least from a moral standpoint. God didn't have to save you. He doesn't deserve your love because He died for you. He deserves your love and obedience because He is God.

How much more, then, ought we who know the truth of the resurrection to be infinitely grateful! We who already owed unto God everything owe unto Him His very life-- except that it has been given freely.

Because quite frankly, we see the Christ and Jesus as entirely distinct entities. We believe in salvation through the Christ--but the mechanism of salvation is that we enact his teachings and so avoid destroying ourselves. We believe in the Christ--but that Christ is an abstraction to be pondered upon and worked towards, not something that was manifested in the person of Jesus (though he came pretty damn close) or anyone else.

We call ourselves "Christians" because we follow what we think is the Christ; it's just that how we conceive of the Christ is different from how you conceive of it.

I'm going to go on record here as a "sort of" liberal Christian. Although there are no Bible citations to support my belief, I will say that I think God is infinitely loving, and that he does not want any soul to perish in the lake of fire.

To that end, I believe that after one dies, one has a chance to see God's revelation revealed to him and one last chance to accept Jesus' sacrifice as atonement for his sins. If he still chooses to rebel and refuse the offer of salvation, then he most certainly will be damned to eternal torture in hell. It's not God's will, it's his will that does it. At that point, it is out of God's hands, only a choice by the person/soul who has died a physical death.

Only the truly rebellious souls go to Hell. Like Satan and all his angels. And the forces of the enemy are constantly trying to influence the actions of people and their attitudes while in the physical plane of existence, to try and harden their hearts to God's love and to feel cynical towards it.

But God has the last word in that matter...

This way, all the people who never heard the Gospel of Jesus Christ and the Good News have a means to enter into heaven. Even Bhuddists, Hindus, Jews and Muslims can enter into heaven if they accept Christ's sacrifice as atonement for their sins in the physical world after having the plan of salvation revealed to their soul-beings after death.

I don't know if that's how it really is, because it doesn't matter to me, as I've already accepted Christ's sacrifice as atonement for my sins

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.