When you hear the term freethinker, what image comes to mind? For me, it is an image of someone who bucks conventional wisdom, goes his or her own way, and questions authority. But I don't think any of this completely captures the mark of a true freethinker, at least not how I think of it. The mark of a true freethinker is that the person is every bit as likely to question the position of his or her allies as his or her enemies.
I know full well that many in the atheist community use "freethinker" as a somewhat more socially acceptable alternative to "atheist" or as an umbrella term that can be used to describe all atheists, agnostics, secular humanists, etc. I've done this myself, and I'm not quarreling with either of those uses here. This post isn't about semantics or definitions (for that discussion, see Austin Cline's post). I'm saying that when I think of what makes someone a true freethinker, what comes to mind is someone for whom critical thinking and meaningful inquiry are more important than group allegiance.
For me, the person who genuinely deserves to be regarded as a freethinker is someone who is willing to ask tough questions of those with whom he or she associates. The progressive who joyfully criticizes conservatives but defends progressives who advocate policies which would be unacceptable if held by a conservative is not much of a freethinker. An atheist who gleefully mocks religious believers but refuses to ask tough questions of his or her fellow atheists is not much of a freethinker.
A genuine freethinker is someone who pursues truth wherever it leads, recognizing that this will often prompt negative reactions from others. Such a freethinker is not necessarily immune to such reactions, but neither will he or she be dissuaded by them.
Personally, being this sort of freethinker is one of the goals to which I aspire. I am certainly not there yet, although I do have my moments. Even though I've become much more of a loner in recent years, it still matters what others think of me. There are still too many times when I hold back for fear of disappointing someone or getting into a feud of which I want little part. But I have made progress in this direction, and that is part of what keeps me going.