The antonym of freethinker

Atheist Dan Barker said:

Freethinkers reject faith as a valid tool of knowledge.

Taking it as it is, what would be the opposite of freethinker in the above context? Restricted-thinker, constrained-thinker, something else?

Solution 1:

I would suggest:

rigid thinkers

as a direct antonym, or

dogmatists

if you want to emphasise that they are adhering to a particular predefined set of beliefs.

Solution 2:

I would say the opposite of a freethinker is a conformist:

[Merriam-Webster]

noun
: one who conforms : a person who behaves in accordance with prevailing standards or customs and typically dislikes or avoids unconventional behavior
// I'm running with the herd and swimming with the tide and being a good little conformist.
— Florence King

adjective
: following or seeking to enforce prevailing standards or customs : opposing or avoiding unconventional thinking and behavior
// The original Beats rebelled against a postwar society they saw as staid, conformist and commercial.
— Jeff Giles and Michael Rubiner

Although freethinker itself does not have a specifically religious context, if you want its opposite in that context, then consider adherent:

: one who adheres to something: such as
a : a follower of a leader, party, or profession
// Freud's adherents
b : a believer in or advocate especially of a particular idea or church
// adherents of Christianity
// adherents of socialism