Freethinker: a person who forms his or her own opinions about important subjects (such as religion and politics) instead of accepting what other people say – Merriam-Webster Dictionary.

Freethinkers maintain that truth should be formed on the basis of logic, reason and empiricism, rather than authority, tradition, revelation of other dogma. Freethinkers include atheists, agnostics, secular humanists and rationalists. No one can be a freethinker who demands conformity to a bible, creed, or messiah.

In his 1944 essay "The Value of Free Thought" Bertrand Russell wrote: 

“What makes a freethinker is not his beliefs but the way in which he holds them. If he holds them because his elders told him they were true when he was young, or if he holds them because if he did not he would be unhappy, his thought is not free; but if he holds them because, after careful thought he finds a balance of evidence in their favour, then his thought is free, however odd his conclusions may seem”.

The whole first paragraph of the essay makes it clear that a freethinker is not necessarily an atheist or an agnostic, as long as he or she satisfies this definition:

“The person who is free in any respect is free from something; what is the free thinker free from? To be worthy of the name, he must be free of two things: the force of tradition, and the tyranny of his own passions. No one is completely free from either, but in the measure of a man's emancipation he deserves to be called a free thinker”.

In another essay, ‘Free Thought and Official Propaganda’, Bertrand Russell suggests that ‘free thought’ has two senses. In its narrower sense it means thought which does not accept the dogmas of traditional religions. In the wider sense, which he regards as of greater importance, it is thought which is not subject to some external compulsion and outward control. This could be though legal penalties (e.g. blasphemy laws, laws against ‘offensive’ opinions), economic penalties and distortions of evidence. He says: 

“it is clear that thought is not free if the profession of certain opinions makes it impossible to earn a living. It is clear also that thought is not free if all the arguments on one side of a controversy are perpetually presented as attractively as possible, while the arguments on the other side can only be discovered by diligent research”. 

In other words, thought is free when it is exposed to free competition among beliefs. If we take all the restrictions he has mentioned – the force of tradition, emotion, legal and economic forces, and biased public opinion – then our task is to make thought as free as possible by combatting these obstacles to it.