Why do most atheists consider themselves liberal (in the American, left-wing sense) ? The liberals and the conservatives fulfill the roles of "rebels" and "establishment", and so people think it's some kind of meaningful choice. Atheists probably tend to be liberal simply because they are more freethinking, and thus they align with the "rebel" stereotype.
While there are fanatics, I doubt most of them honestly believe in liberalism. All the atheists I know who thought they were liberals quickly saw that they weren't, after a small conversation. Of course, it's a very small sample. But it wouldn't surprise me if most atheists were liberal simply out of stereotype or convention.
Is liberalism "freethinking" ? Quite the opposite. To think freely is to set oneself apart from collectivist belief systems based on dogma or emotionalism. But liberalism is a collectivist belief system based on emotionalism. So is conservatism, for that matter, but most atheists already understand why conservatism must be rejected, so I won't expand any energy in that direction. Rather, I'll concentrate on liberalism.
To define it, liberalism is a political position characterized by syndicalism, the Welfare state (based on forced redistribution of wealth), tribalism and support for "environmentalism" (meaning : subordinating society to nature). Its ideological underpinnings are utilitarianism, "common good" collectivism, cultural relativism, and pseudo-science.
Christianity and liberalism are both dedicated to an unrelenting, organized attack against scientific progress and its strict adherence to the facts. To the Christian, this adherence to facts (in favour of Neo-Darwinism, stem cell research, abortion and euthanasia vs the "soul", secular values instead of the Ten Commandments) goes in the face of blind belief in a deity. To the liberal, this adherence to facts (in favour of GM foods, technology vs tribalism, climate science vs liberal pseudo-climatology, against the bungling of studies to support government regulation such as anti-smoking laws) goes in the face of blind belief in the "common good". In the most extreme form of liberalism in the academia, post-modernism, this is manifested as a wholesale rejection of science in favour of inter-personal, racial, feminist, or Marxist "liberation".
Both have no problems with pious lying or violence in the name of their "ideals" (liberal greenie activism right now is considered the most dangerous terrorist movement in the Unites States - groups such as the Animal Liberation Front (ALF), the Earth Liberation Front (ELF), and Earth First!).
Christianity is founded on the sacrifice of individual values in the name of an almighty God/faith. Liberalism is founded on the sacrifice of individual values in the name of an almighty state/democracy/"common good". The only difference is that religion is often spurred by personal experience and is thus generally more personal in nature than political beliefs. But this is a relatively small difference in the face of their gigantic similarities. To most of the reasons to be religious (except the "afterlife"), one can draw a similar liberal parallel.
Insofar as the goal of evangelism should be to free the individual to express his personal values, the consequence is obvious : we should also try to free people from the liberal belief system. If we are to define the most basic worldview based on freethought, as I think most atheists accept, we have to add liberalism and conservatism to the list of dogmas to reject, and preach that the individual should be free to express his values without being coerced by social institutions - whether that institution is a church, a castle or a parliament makes very little difference for the end result.