I'm pretty sure that Martin Luther was considered liberal when he wrote his 95 theses in 1517.
Liberal is a relative term, as others above have pointed out.
I like the terminology developed by Marcus Borg in his book,
The Heart of Christianity. He describes two Christian paradigms which he calls the Earlier Paradigm and the Emerging Paradigm. Both paradigms take very seriously the centrality, truth and reality of God, Christ, and the Bible in the Christian's life. However, they are distinguished by different approaches to these central aspects of faith. I think it's important to point out that these paradigms are more like poles and that Christians are not flat sterotypes that fit one description or the other, but fall somewhere in between. Having said that though, I think it's also apparent that much of the arguing we see among Christians is caused by our closer identification with one pole or the other, and not seeing the spectrum or the relationship between our beliefs.
The Earlier Paradigm views the Bible as a divine product with divine authority. The Bible is interpretted literal-factually and is seen as a revelation of doctrine and morals. There is a major emphasis on the afterlife and what is the right way to believe or do to be saved.
The Emerging Paradigm views the Bible as a human response to God. The Bible is interpretted historically and metaphorically and its function is sacramental and metaphorical. There is a major emphasis on transformation in this life through relationship with God.
As I said, these are poles of a spectrum. I see myself somewhere in between these poles, probably closer to the emerging paradigm. It's also interesting to note that the Earlier Paradigm is actually not the same as the Orignial or Earliest Paradigm, or what we can know or recreate about the earliest Christian communities. The Emerging and Earlier paradigms described above are twins born of the Enlightenment when Biblical criticism and a scientific view of the world started to gain precedence. It's not possible, IMO, to return to the Christian faith exactly as it was before the Enlightenment, when it was possible to believe in the literal events of the Bible without suppressing reason or being credulous. It is possible, however, to have deep faith with reason and also to be loving and connected to people who may be at a different place on this spectrum of belief.
One key part of this, IMO, is not to necessarily agree with each other, but to at least talk to each other with respect and courtesy. Also, I think it's very possible to worship and work side by side, even when we disagree.
You may say I'm a dreamer, but I'm not the only one. I hope some day you'll join me, and the world will live as one.