The assumption that we are infallible can we justify the suppression of opinions we think false. Ages are as fallible as individuals, every age having held many opinions which subsequent ages have deemed not only false but absurd.
― John Stuart Mill, On Liberty

It is indeed the sense that we have infallible knowledge that leads to the suppression of human thought and ideas. One of the greatest sources of that feeling of infallibility is, of course, religion. How can it be wrong if God said it?
One of the great attributes of the scientific method is, in fact, humility, the idea that while we may be on the right track, any particular idea could be wrong in part or in whole. In science, one theory can refute another, so all theories have to be considered.
Political theorists such as Mill applied this to the public sphere and enthroned freedom of speech as a natural right so that everyone can have the opportunity to refute the “false and absurd” ideas of the age, even if those ideas are dearly and strongly held by a majority of people.