Perhaps the oldest classification of freethinker is that
skeptics. (The ancient Greek term skeptikos simply
meant "to inquire, to examine, to consider.")
those who question, who doubt, who challenge the claims of factuality in
accepted beliefs and faith statements. When you make an assertion of
truth, you can depend on any skeptics in your audience to seek clarity
in definition, consistency in logic, and adequacy of evidence for
whatever it is you say.
In ancient times a skeptic would be willing to examine any
assertion of truth, but modern skeptics are not systematic in the
ancient sense (of Pyrrho of Greece) because they trust scientific
research and action based on research findings. They will disbelieve
what science has demonstrated to be false, and they will suspend
judgment concerning whatever has not been checked.
One can perhaps best characterize present-day skepticism
as "methodological." Today’s skeptics insist that claims
of factuality, both religious and nonreligious, must be tested by reason
employing the rules of logic, and by the best tools of scientific
investigation. They will believe, at least temporarily, whatever has
passed the requisite tests, and they will willingly give up whatever
beliefs prove groundless.