Your reason is now mature enough to examine this object [religion]. In the first place divest yourself of all bias in favour of novelty & singularity of opinion. Indulge them in any other subject rather than that of religion. It is too important, & the consequences of error may be too serious. On the other hand shake off all the fears & servile prejudices under which weak minds are servilely crouched. Fix reason firmly in her seat, and call to her tribunal every fact, every opinion. Question with boldness even the existence of a god; because, if there be one, he must more approve the homage of reason, than that of blindfolded fear.

Scan of the original page at The Library of Congress. - Letter to Peter Carr (1787)