Roger E. Olson, an evangelical, has a fine summary of liberal theology in his post titled "What is theological liberalism?" Most people probably thinks it means being pro-abortion, or in favor of gay marriage. Actually, theological liberalism refers to a post-Enlightenment, 19th century, movement in European protestantism. Olson gives the details. His summary point was this:
I think the liberal theological ethos is best expressed in a nutshell by liberal theologian Delwin Brown (a convert to liberal theology from evangelicalism) in his dialogue with Clark Pinnock in Theological Crossfire: An Evangelical/Liberal Dialogue. There Brown asks THE CRUCIAL QUESTION of modern theology: “When the consensus of the best contemporary minds differs markedly from the most precious teachings of the past, which do we follow? To which do we give primary allegiance, the past or the present?” Brown rightly gives the evangelical answer: “We ought to listen to the hypotheses of the present and take from them what we can, but ultimately the truth has been given to us in the past, particularly in Jesus, and the acceptance of that is our ultimate obligation. Everything the contemporary world might say must be judged by its conformity to biblical revelation.” (Of course evangelicals differ among ourselves about WHAT biblical revelation says, but all evangelicals agree that the revelation of God given in Jesus and the biblical message takes precedence over the best of modern thought WHEN THERE IS AN UNAVOIDABLE CONFLICT between them.)
Then, Brown speaks for all liberal theologians when he gives the liberal answer to the crucial question: “Liberalism at its best is more likely to say, ‘We certainly ought to honor the richness of the Christian past and appreciate the vast contribution it makes to our lives, but finally we must live by our best modern conclusions. The modern consensus should not be absolutized; it, too, is always subect to criticism and further revision. But our commitment, however tentative and self-critically maintained, must be to the careful judgments of the present age, even if they differ radically from the dictates of the past.” (p. 23)
Neither one of those is me--in fact, of the two statements, I'd tilt more toward the evangelical view as it is expressed here, a view, incidentally, which seems somewhat more "liberal" than the views of many of the evangelicals I know.
My view is that the tools of historical investigation--the historical-critical method--makes the life of Jesus more intelligible and enables people to hear the true Biblical message. Granted, you have to give up Biblical inerrancy--no big loss in my case since I never fell for that one in the first place.
You also have to give up gnostic and docetic views of Jesus, i.e. that he was fundamentally different from us. Gnosticism and docetism are supposed to be heresies, but most Christian traditions are, in effect, gnostic and docetic in their language and practice. (Any time you tilt more toward Jesus' divinity and away from his humanity, you're moving in that direction.)
Giving up heresy is a good thing. The pre-Easter Jesus was a human being like any other human being. He was raised in a specific context. He lived in a specific cultural and religious environment. He lived at a certain historical moment, and was affected by the same currents of history as everyone else living at that time.
For example, Herod the Great's son, Herod Antipas, inherited rule of the region of Galilee and Perea, both areas with strong Jewish majorities. Herod's "judaism" had always been suspect. The family was originally from Idumea and had converted to Judaism only a few generations prior. (Herod's family was converted during the only period in Hebrew history in which they proselytized. Shows you what proselytizing will do for you.)
It is quite possible that the reason Herod Antipas married Herodias, his brother's wife, is because she was related to the Hasmonean priesthood and would "shore up" his Judaic credentials. It was that marriage that triggered the protest of John the Baptist, which resulted in the Baptist's death and the rise of the Jesus movement.
Jesus didn't float around in some kind of holy drizzle while the Mormon Tabernacle Choir sang the "Hallelujah Chorus" in the background. He was a human being who lived in a certain matrix, as John Dominic Crossan calls it, of culture, religion, social class, economics, and politics.
So yes to the historical method of Biblical interpretation, and yes to scientific exploration as well--and archaeology and anthropology and paleontology and linguistics and all the other sciences. We should love the Lord with our heart, soul, and mind, after all. (Jesus added that, by the way. The original formula doesn't include "mind.")
Plus, Jesus encouraged his people to think. "Why don't you decide for yourselves what is right?" he asked the people (Luke 12:57). He often would ask, "What do you think?" He treated people as if their opinion mattered even as he (usually) corrected them and taught them something new.
This is why I consider myself a "free catholic" and not a "liberal"--(nothing against liberalism, which, for most people, would be an improvement). To take the "catholic" part of that first, I'm "free catholic" because I think that historical investigation of the scriptures supports the catholic faith.
The Christian faith is not about believing the Bible to be historically true in every factoid. The Christian faith is about Jesus the Lord as "Savior of the world." The scriptures make that proclamation and the historical study of the scriptures supports that witness
Admittedly, however, it does complicate it. The Biblical witness is seen as much more varied and diverse than we once had thought. This seems much more historically likely, however, than the pious version of history in which everybody believed exactly the same thing, all the time, forever and ever amen. For the first hundred years, nobody had a New Testament. They had reports and rumors, from more or less reliable sources, and plethora of opinion.
Various arguments pop up in scripture itself. Paul and Peter were on opposite sides. Paul and James cannot be reconciled. Matthew seems to me to be something of a corrective on Paul. The fourth gospel wants you to know that they think different from Peter. It was kind of a mess actually, but it was a glorious mess!
Note also the "free" part of "free catholic." "Free" means thinking for yourself. As Dietrich Bonhoeffer said, we now live in "a world come of age," a world in which people think for themselves and take responsibility for themselves.
Bonhoeffer considered this a good thing, but worried that certain factions of the church would try to keep people in a "religious consciousness," i.e. dependent on religion for answers. The clergy would use their "clerical tricks," he said, in order to keep people in a "religious consciousness." I'm with Bonhoeffer on this. That makes me a "free catholic."