For the purposes of this article, "liberal Christianity" will refer to someone who maintains values which are rebuffed or denounced by the Bible, and yet still identifies as a Christian.
To give a quintessential example, consider the following:
Leviticus 20:1, 13
1 And the LORD spake unto Moses, saying,
13 If a man also lie with mankind, as he lieth with a woman, both of them have committed an abomination: they shall surely be put to death; their blood shall be upon them.
The above passage states unambiguously that both participants in male homosexual intercourse are to be killed, and it represents it as a command directly spoken by God. There are other passages in the Bible which teach the same extremely negative view of homosexuality (Leviticus 18:22, Romans 1:24-28: 1 Corinthians 6:9-10: 1 Timothy 1:10), including indicating that it is a practice which sends one to Hell. This doctrine is never qualified at any point (e.g. that it is only in reference to abuse), nor is same-sex intercourse ever given anything resembling a positive mention, anywhere in the Bible's 1000+ printed pages.
Therefore, if someone does not believe that God was just to demand the unqualified death penalty for homosexuals, or if they believe that such a command is evil or immoral, they would be a "liberal Christian", under the definition discussed above. The same could be shown for those who do not condemn transgenderism (Deuteronomy 22:5), do not support patriarchy (1 Corinthians 11:3, 14:34-35, Ephesians 5:22-24, Colossians 3:18-19: 1 Timothy 2:11-14, Titus 2:4-5: 1 Peter 3:1-7), do not condemn remarriage after divorce (Matthew 19:3-9, Mark 10:2-12), reject the death penalty (Genesis 9:5-6, Numbers 35:30-33 [cf. Revelation 6:9-10], etc.), and so on.
An extremely common liberal Christian perspective is to in effect pretend that everything awful in the Bible is cordoned off in the Old Testament, and that Jesus reformed - or more properly, corrected - the religion to do away with all which is intolerant, essentially leaving only the Golden Rule (Matthew 7:12). However, this is a distortion of the message of the New Testament.
Jesus of Nazareth was a Jew, who affirmed the Hebrew Bible as coming from God. As such, he would have defined his morality by that law. Which means that had he been alive today, his picture of the "Golden Rule" would have included advocating for the death penalty for homosexuals. That is what he believed the law of God commanded, which he constantly quoted, upheld, and affirmed (Matthew 5:17-18, etc.).
He preached the message of a coming "Kingdom of Heaven" (Matthew 4:17, 10:7), which was to be a theocratic state ruled by a religious figure who would have demanded absolute obedience (Matthew 3:1-12). That state presumably would have, as the state of Israel during the time of Moses, as commanded by "God", killed all dissenters (Deuteronomy 13:1-10, 17:2-7). Or, Jesus will have killed all dissenters ahead of time so that they are already in Hell when he starts to rule (Revelation 19:11-18, etc.).
So, if Jesus of Nazareth were alive today, he would immediately reject all "liberal Christians", and probably threaten them with Hell, as he was wont to do (Matthew 5:22, 10:28, 18:9, 25:41, etc.). That is the irony of going to Jesus to escape the implications of the Old Testament. Liberal Christians find no friend in Jesus, because he affirmed the Old Testament, and was generally speaking as intolerant as (or even more intolerant than) any other Jew living in the first century, which modern liberals would find abhorrent.
Another common liberal Christian perspective is to view the Bible as an evolution of human moral understanding. Sometimes, Jesus is represented as the moral pinnacle or zenith. Other times, even Jesus is said to have had a flawed understanding of morality.
However, this is functionally indiscernible from a non-Christian's view of the Bible, who may see moral evolution in the humans who produced it (though the New Testament's fixation on exclusivism [John 3:18, etc.] and hellfire is the opposite of moral evolution, from a liberal point of view). And, the Old Testament represents its morality as coming from God, which in this view, would be false, and slanderous. Finally, Jesus himself, as discussed above, both affirmed the Old Testament generally, and himself taught things which liberal Christians would disagree with. For such a person, "Christian" has lost most of its meaning, as they do not follow the person Jesus "Christ" in any meaningful way, and would be rejected by him if they were to meet him.
Finally, another common liberal perspective is that anything in the New Testament alleged to come from Jesus which contrasts with liberal morality or liberal sensibilities is in fact a corruption. This way, Jesus is preserved as a heroic reformer - in spite of what later generations attempted to put into his mouth, or the machinations of Paul, who was clearly quite conservative in his beliefs, and went on to dominate the Jesus movement.
However, this is creating a Jesus in one's own image. There is just as much evidence that Jesus was every bit the intolerant Jewish rabbi presented in the New Testament as there is that he ever said the Golden Rule. The sources cannot be parsed with the criteria being, "keep that which agrees with liberal sensibilities". That is not dealing honestly with the text. If Jesus really had been a first century liberal, very few would have followed him, and the accusations brought against him, and the movement which followed him after he was executed, would have looked very different.
The essence of liberal Christianity is to give fealty to a man (Jesus) or men (the Jewish prophets) who, by any honest accounting of history, totally rejected their morality on major points, and taught morals which liberals consider abhorrent. That is why is it an illogical position.